Can Germanic religion become the dominant religion of Europe?

With a POD of no earlier than the 3rd century CE, can Germanics make their religion the majority religion in Europe? Basically, imagine a kind of Odinic Europe instead of Christian Europe.
 
The way I understand it: Only great warriors went to Valhalla, everyone else was sent to Hel. Christianity offered the possibility of a good afterlife for everyone.
 
Majority with a millennia long process sure, but I'm not sure it can fit the place that Christianity had with its hegemonical and universal status.

Now I'm not sure how unified the beliefs of the Germanic people from Scandinavia to Alemannia to Gothic Dacia were but you do have more than enough people to base yourself on that, but I think you need to avoid having some sort of unified hegemonic belief in Rome to avoid it spreading and directly challenging the basis for an organized religion prior to the formation of a unified belif.
 
With a POD of no earlier than the 3rd century CE, can Germanics make their religion the majority religion in Europe? Basically, imagine a kind of Odinic Europe instead of Christian Europe.
No. Even admitting there was a "Germanic religion" (which, of course, wasn't the case : we're rather talking of a whole set of related beliefs and practices, which various different takes between peoples), they would simply not have the sheer cultural and political gravity doing so.

If the migration period still happen as IOTL, it's still likely that peoples whom identity and institutional frames are products (more or less original) of the Roman borders (such as Goths, Franks, Alamans, Burgundians, etc.) will still be Christianized out of sheer integration and encadrement trough Roman cultural and social frames, if Christianity is still adopted as main and institutional religion.
If you manage to prevent Christianity of the empire, however, it's still likely that the religions and cults that Barbarians would adopt would be essentially roman-issued : IOTL they already adopted a lot of roman features even in Barbaricum in their religious approach, the best illustration there would be Mars Thincsus (Mars of the Thing) found both in England and Denmark.

Basically, peoples born out of Roman influence projection in Barbaricum and entering then settling Romania would be still largely Romanized.
 
I don't think so. Although Paganism hadn't died out, people in Europe and the Mediterranean were searching for a savior type religion. That's why the cult of Isis (no relation to the terrorists) was so prevalent and a center in Apuleius' Golden Ass.

I don't like saying things are impossible, but it's at least unlikely, I think. Hell, the Germanic invaders tended to adopt the customs of the conquered more than the other way around.
 
No. Even admitting there was a "Germanic religion" (which, of course, wasn't the case : we're rather talking of a whole set of related beliefs and practices, which various different takes between peoples), they would simply not have the sheer cultural and political gravity doing so.

If the migration period still happen as IOTL, it's still likely that peoples whom identity and institutional frames are products (more or less original) of the Roman borders (such as Goths, Franks, Alamans, Burgundians, etc.) will still be Christianized out of sheer integration and encadrement trough Roman cultural and social frames, if Christianity is still adopted as main and institutional religion.
If you manage to prevent Christianity of the empire, however, it's still likely that the religions and cults that Barbarians would adopt would be essentially roman-issued : IOTL they already adopted a lot of roman features even in Barbaricum in their religious approach, the best illustration there would be Mars Thincsus (Mars of the Thing) found both in England and Denmark.

Basically, peoples born out of Roman influence projection in Barbaricum and entering then settling Romania would be still largely Romanized.
Does one strictly need to convert the whole of Romania to achieve a dominant status Europe-wise without Christianity? Taking in consideration the territory from the Vistula and lower Dnieper up to the Rhine(and I guess Germania inferior) and the Pannoian Basin, it seems to me as if you could achieve a dominant status within this area at first, with important communities in the Balkans and immediately over the Rhine. It would be Roman influenced or mixed, but if it's practiced mostly by Germanic people it could be effectively called so, for example I don't think most people would think of Christianity as a non-European religions based on its origins.

Now the problem would be creating this coherent belief system, I guess you need a state to arise that is based mostly outside Romania or in places least Romanized or depopulated that takes over a lot of the territory beyond the limes. Mid term it could look geographically like this(red and orange indicate intensity), it's not strictly speaking all belonging to a unified religion, but to related beliefs which could converge if favourable circumstances arise:

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Well now the question is how "Germanic" this religion would be if one can even measure such things, but I mean if OP doesn't want anything more than "Odin" being the central figure, I don't think we need to strictly addess that issue outside the extreme case that this religion is just Roman polytheism rebranded, which it might as well become but long term you could have it formally differentiated.
 
but I think you need to avoid having some sort of unified hegemonic belief in Rome to avoid it spreading and directly challenging the basis for an organized religion prior to the formation of a unified belif.
That alone wouldn't be enough : the sheer influence from Rome was enough we see roman influence on Barbarian's religions, practices and beliefs even in classical Antiquity.
I mentioned "Mars of the Thing", but there's the importance of Roman subsides and trades in "tools of religion", so to speak (as it happened in late Latenian period). It's as well possible (without any certainty) that the increased priestly role of kingship might comes from this.
Of course, we're talking of a mostly indirect influence, reinterpreted and appropriated by Germanic peoples, and not nearly in the same scale than what happened in Gaul or Britain. Still, the Germanic culture (including religions) was not on an equal standing with the Mediterranean-based Roman civilization in their relationship and more Barbaricum would be opened to Romania (or Barbarians going into Barbaricum), more it would be obvious.
 
Germanic religions can not be dominant. Organized religion like Christianity will overcome it. And besides, Germanic tribes had lesser number of population than the regions they settled in following the fall of the Western Roman Empire. So, no organized religion, no dominant numbers to assimilate will result in nothing.
 
Does one strictly need to convert the whole of Romania to achieve a dominant status Europe-wise without Christianity?
Essentially? Yes.
The cultural and economical core of Europe was based on Mediterranean basin and remained so until a really late date.
Historically, northern Europe was under a constant, if mostly indirect (you still had a lot of cultural-prolitical projection) influence from this region on too many layers that escaping it wouldn't be as much impossible than self-destructing.

We could take the example of Druidism to point how it's possible for any sophisticated society to undergo important religious change and autonomously so (even if Greek influence shouldn't be underestimated), and how something as trivial as commercial monopoly and exportation of a cultural way-of-life can engender it to the point of irrelevance.

Taking in consideration the territory from the Vistula and lower Dnieper up to the Rhine(and I guess Germania inferior) and the Pannoian Basin, it seems to me as if you could achieve a dominant status within this area at first
Dominant positions within the Barbaricum remained largely tied to relationship with Rome and Constantinople, as Hunnic or Avar hegemony can point.
The whole point of Roman power projection in Barbaricum was eventually to prevent such dominance to emerge without their consent : it was far from systematically efficient I wholly agree : but then, if Rome was unable to press its interests, Barbarians generally went to the juicy, prosperous Mediterranean-based Romania, not undergoing a Drang Nach Osten.
It really goes down to this : a Roman Empire too weak to prevent hegemonic and complex build-up in the Barbaricum is bound to see this turned against it.

but if it's practiced mostly by Germanic people it could be effectively called so, for example I don't think most people would think of Christianity as a non-European religions based on its origins.
To be on the level with you, this comes from nowhere : nobody yet argued about ethnicity as a relevant factor.
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I could frankly reverse the proposition : if you twist the religious built-up just enough to be adopted by Romans, it would cease to be a Germanic religion, just as Christianity is no longer a semi-apocalyptic Jewish cult.

Now the problem would be creating this coherent belief system, I guess you need a state to arise that is based mostly outside Romania or in places least Romanized or depopulated
Problem being, of course, that chiefdom build-up in these regionis directly related to Roman presence, influence, trade, relations, etc. Not that autonomous build-up is unheard of course, but it generally involves largely isolated areas : I'm not too sure about chances of widespread religious changes (which can happen quite easily, and more progressively than "reformed paganism" we agree on this most certainly).
Isolation and Build-up are not necessarily multually exclusive, but in this context...

but I mean if OP doesn't want anything more than "Odin" being the central figure, I don't think we need to strictly addess that issue
Well, we might have to even in this case : while Woden/Odin was an important deity for Germanic peoples, it might not have been predominant among all of them (for instance, Goths or Saxons)
Now, the reasons for his promotions are at best speculative, so I'm not really convinced with explanations tying it to the consequences of militarisation and build-up of Barbaricum from the IInd century onward.

Personally, but I'd rather ask someone knowledgeable about this for advice, I'm not sure how viable would be the promotion of a prime divinity : I'd lean in favor of a rough henoetism comparable to what existed in late Rome (note that I don't think that it would be a direct model) differing from region to region but still coherent enough to be considered as one defined ensemble.

EDIT : Just to point that I don't think the emergence of a specific Germanic religion is impossible on the long run : there's too many possibilities in Ancient and Late Ancient history to write it off, IMO.
But the problem arises more when you ask yourself what to do with Wotanism, Tyrism, Hornedhelmetism, etc.once it emerges :how to widespread to richer, more central regions without turning it as something culturally and theologically different to the point it can't be considered a Germanic religion (again, not ethnically but culturally).

There's the case for Islam, whom religious expansion didn't involved great departures from its cultural basis, and that just did well, I admit. We could then assume the possibility of "Gemanism" doing something similar enough, but I'm quite dubious : proto-Islam for all its particularities, emerged as a relatively structured doctrine (and refined after the early conquests) because it borrowed heavily on pre-existing ones and wasn't an evolution from Arabic polytheism.
 
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Essentially? Yes.
The cultural and economical core of Europe was based on Mediterranean basin and remained so until a really late date.
Historically, northern Europe was under a constant, if mostly indirect (you still had a lot of cultural-prolitical projection) influence from this region on too many layers that escaping it wouldn't be as much impossible than self-destructing.
Well I wasn't thinking about a short term process, obviously it would take much more time than it took Christianity to become majority or dominant in Europe and considering we are talking about a religion that didn't even exist IOTL, it would be a multi step process.

Dominant positions within the Barbaricum remained largely tied to relationship with Rome and Constantinople, as Hunnic or Avar hegemony can point.
Would a Barbarian takeover of the East help on that front(in the sense of helping the creating of more independent entities), especially if we have no Huns around?

The whole point of Roman power projection in Barbaricum was eventually to prevent such dominance to emerge without their consent : it was far from systematically efficient I wholly agree : but then, if Rome was unable to press its interests, Barbarians generally went to the juicy, prosperous Mediterranean-based Romania, not undergoing a Drang Nach Osten.
It really goes down to this : a Roman Empire too weak to prevent hegemonic and complex build-up in the Barbaricum is bound to see this turned against it.
Sure but this doesn't have to entail a migration towards Rome similar to IOTL, especially if we are talking about a pre-Constantine collapse. Is it really all(Rome collapsing and as many Germans migrating in) or nothing(no Roman collapse, the post 3rd century limes is preserved)? Is it possible to have either no functional central Roman state or one weak enough to not have the ability to proactively block development of strong Germanic confederations but still have fewer Barbarian migrations deep into Roman territories, especially in the West(Italy, Africa, Spain, Southern Gaul-Italy)?

To be on the level with you, this comes from nowhere : nobody yet argued about ethnicity as a relevant factor.
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I could frankly reverse the proposition : if you twist the religious built-up just enough to be adopted by Romans, it would cease to be a Germanic religion, just as Christianity is no longer a semi-apocalyptic Jewish cult.
It's not exactly purely ethnic, it's more about Roman-influenced Germanic practices becoming eventually a organized religion that is differentiated enough and that exists mostly outside the culture or area where the influence came from.

The scenario I tried to outline doesn't really have millions of Romans convert to this newly formed religion shortly after the collapse of the Roman state, I was thinking something way later than IOTL Islam.

Problem being, of course, that chiefdom build-up in these regionis directly related to Roman presence, influence, trade, relations, etc. Not that autonomous build-up is unheard of course, but it generally involves largely isolated areas : I'm not too sure about chances of widespread religious changes (which can happen quite easily, and more progressively than "reformed paganism" we agree on this most certainly).
Isolation and Build-up are not necessarily multually exclusive, but in this context...
Would general Roman influence or creation of states through said influence on Barbaric territories necessarily prevent the appearance of new religious systems even long term in the future?

Well, we might have to even in this case : while Woden/Odin was an important deity for Germanic peoples, it might not have been predominant among all of them (for instance, Goths or Saxons)
Now, the reasons for his promotions are at best speculative, so I'm not really convinced with explanations tying it to the consequences of militarisation and build-up of Barbaricum from the IInd century onward.
Personally, but I'd rather ask someone knowledgeable about this for advice, I'm not sure how viable would be the promotion of a prime divinity : I'd lean in favor of a rough henoetism comparable to what existed in late Rome (note that I don't think that it would be a direct model) differing from region to region but still coherent enough to be considered as one defined ensemble.
Even having Odin being the most important deity in all communities wouldn't still be enough to call all the different practices a formal unified religion, so making Odin predominant in those populations would be part of the formation and spread of an organized religion I'd imagine.

EDIT : Just to point that I don't think the emergence of a specific Germanic religion is impossible on the long run : there's too many possibilities in Ancient and Late Ancient history to write it off, IMO.
But the problem arises more when you ask yourself what to do with Wotanism, Tyrism, Hornedhelmetism, etc.once it emerges :how to widespread to richer, more central regions without turning it as something culturally and theologically different to the point it can't be considered a Germanic religion (again, not ethnically but culturally).
Instead of having the religion spread to a central place in Romania or generally outside Germanic territories, isn't it possible to have rich central places develop along said territories? I'm thinking about along the Danube or the Rhine, alongside trade routes between the East and the North Sea, looking at this map that I found from one of your older posts, I think the Danube area would be quite good, although we have the problem of the role of Odin in the pagan practices of the Goths:


There's the case for Islam, whom religious expansion didn't involved great departures from its cultural basis, and that just did well, I admit. We could then assume the possibility of "Gemanism" doing something similar enough, but I'm quite dubious : proto-Islam for all its particularities, emerged as a relatively structured doctrine (and refined after the early conquests) because it borrowed heavily on pre-existing ones and wasn't an evolution from Arabic polytheism.
Well Christianity and Judaism were also quite present in Arabia, so on that front it is still an evolution of religions practiced already by Arabs even if there is not as much theological input from polytheism as one would expect looking solely at the pre-Islamic demographics.
 
Germanic religions can not be dominant. Organized religion like Christianity will overcome it. And besides, Germanic tribes had lesser number of population than the regions they settled in following the fall of the Western Roman Empire. So, no organized religion, no dominant numbers to assimilate will result in nothing.
I think the religion would have to be restructured, and I think Julian attempted something similar with Roman Religion, and an attractive alternative offered. So I have a thread about an East German Kingdom (a dear lonely thread that would love some comments to keep it company) situated around Crimea and the Volga. So if you have a stable, relatively large kingdom for it to take root in, then the religion can spread to other Germans or Guals. But the religion won't carry enough people to sway a majority of the Mediterranean urban centers. So you will need to have a situation where they experience depopulation from plague and warfare, but the less urbanized Germans and Gauls survive.
 
I think the religion would have to be restructured, and I think Julian attempted something similar with Roman Religion, and an attractive alternative offered. So I have a thread about an East German Kingdom (a dear lonely thread that would love some comments to keep it company) situated around Crimea and the Volga. So if you have a stable, relatively large kingdom for it to take root in, then the religion can spread to other Germans or Guals. But the religion won't carry enough people to sway a majority of the Mediterranean urban centers. So you will need to have a situation where they experience depopulation from plague and warfare, but the less urbanized Germans and Gauls survive.

I think the problem here is that with Julian, Hellenism already had some of the structure that could evolve into a centralized religion like Christianity (with a lot of effort here.) “Germanic religion” on the other hand had almost none of that. Practices and rites varied from tribe to tribe and only can be classified as a “religion” in the loose cultural and historical respect.

To have a Germanic Julian, they’d have to take the mess of practices and customs, attempt to cobble them into something cohesive, and then try and get most of the Germanic world to accept it against the forces of Christianity or any other religion. And that’s just for the traditionally Germanic portions of Europe. Attempting to “convert” populations of other places in Europe with a thoroughly Germanic religion is not that feasible in my opinion.
 

Philip

Donor
I think the religion would have to be restructured, and I think Julian attempted something similar with Roman Religion, and an attractive alternative offered.

Copying Julian will be difficult. He combined the established popular practices with a strong, well-known philosophical basis. He copied many of the practices that made Christianity popular while applying the organisation of the imperial state. Even then, his efforts were not well received.

A far as I can tell, the Germanics are missing three of these four pieces.
 
Would a Barbarian takeover of the East help on that front(in the sense of helping the creating of more independent entities), especially if we have no Huns around?
We couldn't be much seeing a takeover, than the possible establishment of a similar hegemony if we have enough Roman subsides going around.
Would it help? I don't think so, not as such, we'd be rather into a complex network of redistribution and fidelities, and while I'd expect cutural transmission on the long run, I'm dubious about religious build-up. You'd eventually have similar and as much possible results without a general hegemony.

Sure but this doesn't have to entail a migration towards Rome similar to IOTL
It really implies it : unless we're talking about climatic PoDs, you'd have a general set of migrations to Mediterranean sea with the decline of Roman trade (which supplied Germans with a lot of grain and mobilisation resources).

Is it really all(Rome collapsing and as many Germans migrating in) or nothing(no Roman collapse, the post 3rd century limes is preserved)?
No, you have as well Germans remains on the limes, with migrations patterns basically looking like what happened in the IInd to IIIrd century (very roughly) as Roman partners if not clients. Which doesn't let a lot of opportunities around.

Is it possible to have either no functional central Roman state or one weak enough to not have the ability to proactively block development of strong Germanic confederations but still have fewer Barbarian migrations deep into Roman territories, especially in the West(Italy, Africa, Spain, Southern Gaul-Italy)?
The problem isn't having them settling deep down in the empire : it's that the buil-up of Barbarian confederations even at the limes directly depends from the capacity of Roman state to either buy out peace, to employ Barbarians to limit their advance, or more simply raids as source of welth and mobilisation of such coalitions.

The scenario I tried to outline doesn't really have millions of Romans convert to this newly formed religion shortly after the collapse of the Roman state, I was thinking something way later than IOTL Islam.
One possible problem is that more you wait, more you let parallel and possibly stronger religious evolution in Mediterranean basin getting more strong and more rooted into general culture and identity and, even if by some happenstance, doesn't influence a significantly on Barbaricum (especially a Barbaricum whom own build-up would make it, paradoxically, more connected to the wider world).

Would general Roman influence or creation of states through said influence on Barbaric territories necessarily prevent the appearance of new religious systems even long term in the future?
It doesn't prevents them as much it would tend to make them appear along non-Germanic lines (and possibly as a syncretic belief) and under its religious influence. Or, more simple, it may be not the religion(s) as practiced in Romania, but I doubt it would be recognizable form of Germanic polytheism.
Instead of having the religion spread to a central place in Romania or generally outside Germanic territories, isn't it possible to have rich central places develop along said territories?
Again, how do we prevent the influence from the actual cultural and economical cores of Europe, while these rich places devellop?
 
The way I understand it: Only great warriors went to Valhalla, everyone else was sent to Hel. Christianity offered the possibility of a good afterlife for everyone.
That is only the surface level of the Germanic afterlife.
Hel was reserved for essentially the humdrum, those who squandered their lives. Whilst modern culture has emphasised Valhalla, and certainly Vikings seemed to think it was the best, there are actually quite a few different afterlifes, with half of all warrior souls not even going to Valhalla.

As to the question itself, potentially with a POD that leads to a very decentralised Christianity.
So ive spoken about this a few times, but there is an interesting claim RE conversion tactics of the Germanic peoples. Namely, their were supposedly attempts to find something of a halfway point in their own myth; Germanic mythology being the prior world and the current world being ruled by a single god, who was Baldr of the old world.

The reason I say supposedly is that I have never found difinitive proof of this claim. Largely, ive read it from a few sources, but following the source chain seems to end at inconclusive points. Whilst I cant speak to the authenticity of the claim, there is reasonable precedent in the narrative, namely in the celtic church of Ireland and how the Book of Invasions christianises the many tribal and mythical histories of ireland (i.e. literal biblical figures as founding figures). Its quite an odd duck to have been made from a Christian, because it very clearly promotes the existence of two divinities (Yahweh and Danu) in an almost wiccan fashion.

So, my suggestion presuming the conversion scheme is real, would be to have a decentralised Christianity to the point where when somebody makes such a synthesis, it becomes a germanic religion in its own right (paying tribute to the old gods and the new).
 
We couldn't be much seeing a takeover, than the possible establishment of a similar hegemony if we have enough Roman subsides going around.
Would it help? I don't think so, not as such, we'd be rather into a complex network of redistribution and fidelities, and while I'd expect cutural transmission on the long run, I'm dubious about religious build-up. You'd eventually have similar and as much possible results without a general hegemony.
So the survival of the Roman East is not all that important in this case, would the loss of the European part help the Danubian centers in developing more independently?

It really implies it : unless we're talking about climatic PoDs, you'd have a general set of migrations to Mediterranean sea with the decline of Roman trade (which supplied Germans with a lot of grain and mobilisation resources).
Would you say the further push factor (the Huns for example) was not that important all things considered in determining the scale of the migration?

No, you have as well Germans remains on the limes, with migrations patterns basically looking like what happened in the IInd to IIIrd century (very roughly) as Roman partners if not clients. Which doesn't let a lot of opportunities around.
The problem isn't having them settling deep down in the empire : it's that the buil-up of Barbarian confederations even at the limes directly depends from the capacity of Roman state to either buy out peace, to employ Barbarians to limit their advance, or more simply raids as source of welth and mobilisation of such coalitions.
How long can a period of raids last? Can a Rome which becomes hostile or just less open to the idea of foederati arrangements create a situation where the Germanic tribes can only go so far as raiding or settling locally like they did in the 3rd century? Or at the very least can you have a smaller migratory group(possibly fueled mostly by Inner barbarian groups as opposed to the ones on the limes?)

One possible problem is that more you wait, more you let parallel and possibly stronger religious evolution in Mediterranean basin getting more strong and more rooted into general culture and identity and, even if by some happenstance, doesn't influence a significantly on Barbaricum (especially a Barbaricum whom own build-up would make it, paradoxically, more connected to the wider world).
The question is if without a central state this religious evolution would happen universally or regionally(especially in light of a decline in scale of trade and the general demographic decline)

It doesn't prevents them as much it would tend to make them appear along non-Germanic lines (and possibly as a syncretic belief) and under its religious influence. Or, more simple, it may be not the religion(s) as practiced in Romania, but I doubt it would be recognizable form of Germanic polytheism.
Possibly but in of itself I'm not sure how the heterogeneous influence from Rome would create something clearly non Germanic, especially if it stems from a gradual, multi-century process which sees the cultural background change as well.

As a useful comparison you mentioned the development of druidism and it's connections to Hellenic influences, would you say that Druidism was distinctly different or recognizable in relation to the previous practices in the region or that druidism in of itself is syncretic? Maybe the comparison doesn't make sense but I felt it would have helped having OTL examples to understand.

Also I wonder how religions like Zoroastrianism differ from the previous beliefs present in Iranic lands.
Again, how do we prevent the influence from the actual cultural and economical cores of Europe, while these rich places devellop?
Maybe a decline in the actual existing centers neighbouring those developing ones? Something akin to the Gothic wars in Gaul and Italy or maybe, when taking into account the 3rd century crisis, Northern Italy, the Rhine frontier and the Balkans being subjected to harsh raidings that could fuel the creation of alternative trade routes or a movement of people outside the region towards farther away areas that would subject the Danube and Rhine region to a lesser direct influence as it otherwise would have experienced.
 
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So the survival of the Roman East is not all that important in this case
It is pretty much important: without the East to provide WRE with a modicum of subsides, troops and most of all naval power, WRE is toast and will be taken over by Barbarians as IOTL.

would the loss of the European part help the Danubian centers in developing more independently?
Yes, in the sense that it would imply a Barbarian Italy and its influence over the region. No, in the sense that loosing all of Balkanic area means that you'd end up with Barbarians in Greece and Thrace (possibly hellenophone instead of romance, so I guess that's a noteworthy change enough) pulling the same thing than Barbarians in western Romania. You could say that the Danubian region could be treated the same way than Pannonia during the Vth/VIth centuries, as in a relatively remote strategical client (there undisturbed by ERE) but I don't see a good reason why Goths wouldn't move en masse in "inner" Romania letting the place to other Germanic peoples or even Sarmatized peoples (probably a mix of both) as vulnerable as Gepids in face of outer groups.
It could technically work, but I don't find this particularily plausible either as for the PoD, either for the desired result. Not impossible, you'd be right.

Would you say the further push factor (the Huns for example) was not that important all things considered in determining the scale of the migration?
I think it had a decisive political influence on how Barbarian peoples moved and evolued, but on a demoraphic matter, it's indeed the case that whole areas were depopulated in Eastern Europe trough migrations before Huns came in Europe.Getting rid of the Huns is not getting rid of migrations, but allowing some groups to do better, and some other to act differently.

How long can a period of raids last?
How long Rome can be both vulnerable enough to not prevent these, but strong enough that it doesn't loose political control? It seems to me less as a maintainable situation than something transitional or more or less temporary.

Or at the very least can you have a smaller migratory group(possibly fueled mostly by Inner barbarian groups as opposed to the ones on the limes?)
The scale of migrations is less the problem (we're talking of fairly limited numbers, comparatively-speaking, depending on the size from some thousands to some ten of thousands) than the political capacity to Rome to deal with it, and eventually it more or less worked : ERE noticably did not collapsed, and WRE could have found a modus vivendi as late as the early Vth, maybe later without Hunnic hegemony.
This is precisely one of the possibilities I find quite plausible to "fix" Goths on Danubian basin, on both banks.
Of course, that means Goths being directly under Roman influence : probably not to the point being romanized to the point they were IOTL in Spain and Italy, but enough to be concerned by Rome's religious policies (Homoeianism, Orthodoxy, all that rap). We could agree, however, that it means that further groups in Barbaricum (but less Germans than Balto-Slavic peoples IMO) could form specific religious practices inspired both by their own beliefs, and what would come from the south.

The question is if without a central state this religious evolution would happen universally or regionally(especially in light of a decline in scale of trade and the general demographic decline)
You had, both in Constantinople and Barbarian kingdoms IOTL, a great stress put on religion and its social ties : in fact it was even more important among Barbarian kingdoms (to the point a good part of the institutional and legal arsenal that wasn't issued from Roman legacy was borrowed from the Bible) in search of a structural identity.
Of course with a marked decline of trade (which wasn't enough to limit religious evolution : see the conversion of Anglo-Saxon kingdoms, directly tied to AS courts relations, including and mainly trade, with Frankish Gaul) would play a role into conversion or their absence thereof. Still, a less connected region means as well much less reasons to undergo a cultural change.

Possibly but in of itself I'm not sure how the heterogeneous influence from Rome would create something clearly non Germanic
It's less due to a diffuse, if constant, cultural influence but the combination of just this and direct political influence. Again, regardless of the PoD or the TL, I don't see a good reason why a distinct religion from polytheistic background couldn't emerge : it's rather becoming big enough to become dominant I find particularily implausible.

especially if it stems from a gradual, multi-century process which sees the cultural background change as well.
Which would be along a still existing and almost by definition much more dynamic and stronger cultural influence from european cores. The same way that you didn't have a one Sudanese religion to emerge in contrast with Islam, you'd have all the chance not having a one Germanic religion to emerge in contrast with either Christianity or the whole of post-classical Roman paganisms (on which, I wholly agree there, a Germanic-based religion would have all the chances to be integrated, but as the pretty much popular worship of Isis IOTL, likely romanized).

As a useful comparison you mentioned the development of druidism and it's connections to Hellenic influences, would you say that Druidism was distinctly different or recognizable in relation to the previous practices in the region or that druidism in of itself is syncretic? Maybe the comparison doesn't make sense but I felt it would have helped having OTL examples to understand.
Keeping in mind that we don't know this much about Druidism, and a lot less about what existed before.

Sociologically, Druids were possibly leading scholars issued from nobility having already priestly duties in ordering temples and preparing festivities thanks to astronomy knowledge. Their preeminence, against bards and especially vates (roughly seers and sacrificers) isn't quite clear, if indisputable : generally the IVth century BCE is considered as a good guess for dating its more or less definitive establishment. Greeks account on Druisim seems to avoid the literary tropes about foreign priest/scholars/philosophers (as marginal men) and even seems to capture rather well their social role which could imply a relatively good communication between Greeks and Celts on this regard (relatively to more or less everything of course).
So, it's possible that Druidism emerged as a syncretism of already present beliefs, practices and structures; with a Hellenic influence, notably pythagorism which was quite dynamic in this era, especially in their activity and "ingerence" in social sphere.

Druids never entierely managed (if they even sought) to get rid of vates, bards and religious beliefs lying around, being content to push them back to a secondary role, while focusing on teaching the upper social layers. Which with the growing importance of Gallic polities in the Second Iron Age, marked the appearance of public cult structures, as it seems outside urban communities (while a lot of these communities were gathered around pre-existing cult centers), trough an organized network and regional (and pan-Gallic) assemblies.

We may be talking there, rather than a new religion, of a new religosity not really dealing with old beliefs and practices, than reorganizing them structurally and conceptually, refining these in the process. As it was tied a lot to social structures, when these got weakened at the benefit of Roman influence, so was Druidism which hold best in the IInd and Ist century BCE in places further to Roman influence namely Belgium and Britain where their roles is arguably unclear (possibly mixed with a religious organisation closer to what existed in Gaul before druidism in its strictest sense in the remotest areas).
In the absence of Rome, maybe it could have evolved better, maybe declining but able to recover : but when you have a political, economical and cultural behemoth at your door able to defeat your influence...

Also I wonder how religions like Zoroastrianism differ from the previous beliefs present in Iranic lands.
My poor understanding of the question makes me lean to understand it less as something roughly similar, except on a much more mature social-cultural base, and therefore more able to refine itself quickly and with support of a strong political power.

Maybe a decline in the actual existing centers neighbouring those developing ones?
I think it's hard to consider a perpetual decline of these centers : at some point, either it recovers, either it collapses.
You mentioned the harsh raiding of the 3rd century, but it can be perpetuated for much long without having these provinces being vulnerable to basic settement or political take-over.
 
I think you can, but the religion would need to change some become more formalized. If you have a figure arise who serves as a kind of Mohamed analogue and creates a more formal scriptural version of OTL Germanic beliefs, I don't see why the Germanic conquests couldn't have taken on a religious aspect similar to OTL expansion of Islam.
 
How about Chlodwig 497ish does not convert to the catholic faith, but the arianic faith (after Arius from Alexandria - people of that religion held that Jesus is not God, but was created)?
In a book I recently read, it was claimed that the arianic faith would have led to a more decentralized, local nationalistic church, something that CountPeter suggested above as potential springboard for such a development.
 
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