In what direction would Europe have developed if the Roman Empire had not accepted Christianity as its official religion.
I notice, on a second reading, you are not explicitly saying here that the Empire adopts
no official new universalizing religion, only that that religion is stipulated not to be Christian. And yet most people replying seem to be reading it as the former--no official cult whatsoever, or anyway one that goes no deeper than saying that the Caesar is a living god himself, which is pretty trivial from the point of view of broad faith and doctrine.
I will take up the challenge in both senses--one (that I much prefer as being in my opinion far more likely) that there is an official cult that is adopted widely and deeply in the later Empire, but it isn't Christian, and also try to consider the possibility and consequences of there being none whatsoever.
Is it likely that we still would see the development of a feeling of belonging to a common culture?
Well, there were already in the early Empire, if not one common culture linking everyone universally, than several interlinked networks of widespread common culture across the Empire. For one thing, there was the general dominance of Hellenistic culture throughout the Med and indeed far to the east of what the Empire could ever conquer or if conquered, hold for long. This is not universal in the sense that it didn't go down deep to the local grassroots in many places (though it eventually did in some, Egypt for instance). But there were a whole lot of Greeks spread all over the range of Alexander's conquests and beyond; the Romans for instance though never in that sphere were pretty busily Hellenizing themselves--to an extent anyway. The Greek communities hung on to their identity tenaciously and it it was quite possible for people who were in no way ethnically Greek to also be accepted as Hellenized.
Then there were the Romans themselves, scattered all over, as ruling proconsuls and as legionaries and as colonists (often retired legionaries). They too over time came to incorporate people from other backgrounds as more or less Romanized.
Then there is the network of regional trade all over the Mediterranean that the Empire enabled to go forward largely freed of the old limits of fear of piracy and capricious local policies--to be sure, burdened instead with Imperial taxation and so forth, but on the whole the formation of the Empire led, for a while anyway, to a major surge in general prosperity that helped vindicate the political system. And thus to an extent automatically vindicate the idea of membership in a larger common world. By its nature this trade tended to make ideas as well as goods common and thus was a substrate of general unity.
It is precisely because of the transformation of the pre-Classical local peasant realms by the evolving network of intensifying trade that I believe the old inherited religions of the diverse peoples of the region were challenged and ultimately undermined among at least the richer and more traveled of these people, and why there was a crisis of religion in the Classical world. The Hellenizers and the Romans among them in particular were Seekers; they were looking far and wide, high and low, for a new religion that would more effectively integrate their experiences with their hunger for meaning. I strongly believe that if Christianity had not evolved, to a great extent shaped by these very same Seekers into the thing they desired, then some other of the hundreds of cults the Romans and Greeks pursued would have grown to fill the role instead, and sooner or later the Empire would adopt a definitive doctrine of its own (and as with Christianity, other versions of it, rejected by the central authorities, would also find their niches despite persecution--to an extent, because of it!)
I hardly think this process would fail, but what if it did? I will try to bear in mind that possibility too.
Since this would also butterfly Islam,
Oh, I would not presume that! Of course it would make an ATL Islam analog quite different in detail, in the text of an alt-Koran for instance, but my suspicion is that perhaps the emergence of a strong monotheist religion very very much like Islam is just in the cards, for reasons I hope to get to eventually in this post. It keeps coming up as an unquestioned assumption, but I propose to question it.
Technically of course it wouldn't be exactly OTL Islam so technically of course it is strictly true. Substantially though I won't rule it out.
I assume that Southern Europe would continue to have closer ties to the areas south of the Mediterranean than to Northern Europe (although this would also be likely in a scenario with a POD that only butterflied Islam, not a Christian Roman Empire). Would one still see the development of several smaller kingdoms fighting each other, marrying each other and so on?
Well, sure! Unless you are making a hidden assumption that the adoption of Christianity was the essential reason the Western Empire fell, it seems pretty darn inevitable to me that sooner or later it would break apart for some reason or other, most probably about when and how it did OTL. To be fair I have an essentially Marxist notion about the rise and fall of the Empire; I believe that the same elimination of borders that allowed an immediate surge in prosperity in the early centuries also in the long run undercut the basis of merchant profit, leading to increasing slavery, increasing governmental burdens and dictatorial means of trying to ensure everyone filled their economic roles despite their disincentives to do so, thus undermining the sources of Imperial strength, while at the same time the invasions of Germanic and other peoples were both driven by events in central Asia and by the attraction of the poorly defended western Roman lands. So if all this is so the collapse was inevitable and on much the same time scale too; unless one then envisions a single overlord of all the invaders gaining hegemony over them all and managing to institute it through succeeding generations, the result is of course many kingdoms.
An Empire with a universalizing religion has got more tools to try to fight off this inevitable doom than one without, it seems to me--if somehow or other, the Emperors had never adopted one and imposed it (bearing in mind, imposing it is a matter of finishing a process that has largely gone forward without governmental enforcement and probably early on, despite active persecution against it) then I'd think the collapse would come all the sooner, perhaps by 50 years or a century even.
I suspect part of what shapes European history is geography; that it is rather more difficult to unify that peninsula than other comparably large regions of more or less compatible modes of cultivation and continuity. Note that the Romans, as you observe, only moved north into it to a limited extent.
Would it mean an earlier or a delayed industrialization or would religion not influence this in any way?
Well, again it matters whether the Romans simply adopted some other cult, or whether they somehow failed to do so at all. In the latter case, I suppose Europe would lack an important element of integration. It might be that instead of getting the medium-sized dozens or so nations that contended over the rise of capitalism OTL, we might instead have hundreds of little polities; maybe the mercantile networks that would evolve to link them (even as they war with one another of course) might still rise to a level where capitalism becomes possible, even if they don't enjoy the patronage of moderately powerful states that OTL helped foster vital early stages. Or we might have the dozens of states, with each more or less traditional pagan subgroup consolidating; a few Nordic/Germanic realms here, some Slavic ones there. The fact that they don't have any common religion might not substantially impede the trade--it might however impede mobility of capital and the adoption of common institutions.
But I think this "universal religion never happens" scenario is pretty darn unlikely. If the Romans don't provide it, the evolving Europeans may come up with one of their own. Or Buddhist or my assumed alt-Muslim missionaries may get there first.
The early modern times in which Europe committed to capitalism and hence industrialism (the two being essentially one and the same) are a long long time after the collapse of the western Empire after all. The question of whether there is still no universalizing religion is relevant but I think misguided; the question of whether the Romans provided it or not is mooted by antiquity, unless one supposes that the universal identity had to forged a thousand years before capitalism, or that the capitalist elements could not arise without preserving a substrate of Classical civilization--which may be a point very well taken.
If it would delay industrialization, would some other part of the world industrialize first?
I suppose so eventually, but not sooner. The European expanding capitalist sphere did not start poisoning other wells until it was already well under way, by the 15th century. Well, certainly the Fourth Crusade stuck a knife in the back of the Byzantine Empire, but that was hardly a case of one capitalist murdering another. If Europe were somehow diverted from developing capitalism I might guess it would be many hundreds, perhaps thousands, of years before some other region were transformed suitably to play the role. There are a lot of reasons Europe is favored for the part; the most likely thing I'd guess is that is Europe, but later.
And as with the timing of the fall of the western Roman Empire, I'd bet on it being Europe, within a couple centuries plus or minus, of when it happened OTL.
Is it at all possible to say something about this? Was the world "predestined" to industrialize sooner or later anyway, due to gradual development of technologies)?
Well, I obviously think so!
I feel like it'll be a lot less different than people may believe. Religion and Philosophy, as a culture, both influences its believers and is influenced by their existing cultures.
Yes, I don't think level of development would be significantly different.
Though severely reducing religion as a motivation for bad shit would be interesting- people will find other excuses to go to war with each other of course but still, intruiging.
These are my views essentially though I want to stress, I don't think religion is just a side phenomenon; it isn't like the style of pillars or jar decorations. It goes deep, it is a major social mechanism. All society contends using religion as one of their major tools and battlefields in the class struggle. A successful universalizing religion must evolve to play contradictory roles, serving as a channel and pillar of elite rule while also serving as the custodian of mass discontents. It serves palace and peasant--not alike, but must handle the needs of both somehow to survive. A religion that is all highflown fashionable court philosophy has no traction with the masses; if it is all pagan grassroots it can't engage with the machinery of high society. A universalizing religion somehow manages to mesh both together--the action of the machinery can be and often is violent of course!
In other words, Tyr, I predict there would be some universalizing religion or other, and it would always play a part in providing excuses or even outright create perverse motives for "bad shit." The former is a big part of what it is all about; the latter, a transposed version of the former.
The big thing that interests me and I wish I had the knowledge to develop would be buddhism's westwards spread. It would be quite interesting to see how much of it would be took on board by European cultures to merge with the native faiths.
I'd think that before the Buddhist missionaries got that far west the Europeans would have hit on some version or other of a universalizing religion of their own. But if somehow the process is delayed that long I suppose they would find fertile soil in the west indeed. This puts a time limit on how long Europe can go without a universalizing faith I suppose!
I'm not so sure on the no christianity->no islam-> no Arab invasions chain.
IOTL we see the Arab invasions did happen. There was clearly something happening in Arabia to push it to this. I have trouble believing it was just a religion that was at fault- and even if it was, why did this religion emerge when and how it did?
I've already said I think the first statement you make is quite right; I think something like Islam would come out of Arabia about when it did OTL. As for the second set of statements--well, I do think that when the Arabs did expand they would do so under the banner of a universalizing religion new to them, and if they somehow did not, their expansion would be much more strongly checked. I think the same circumstances that caused the expansion also caused the religion and the two went hand in hand, reinforcing one another.
I think without Islam the Arab invasions would be a lot less succesful but you can see in the centuries leading up to Islam that the Arabs were developing and growing in strength and that something was going to happen.
And that's without even mentioning the reason for their success- the Byzantine-Persian wars.
I'm just saying, the "something" that happens would almost surely involve the new religion. Which would be created to order by the same unbalanced circumstances that cause the secular factors one might try to separate from the religious ones.
I remember one scholar who claimed that with no Christianity things would go much worse for the Native Americans (one the old world made contact with the new world) as there would be no call for mercy towards them. I don't know if this holds up to scrutiny or not
A lot of people have gone back and forth on aspects of this here; IMHO first of all the Europeans would have adopted some sort of universalizing religion before they got that far, and this religion, whatever its origins, would incorporate both injunctions to charity and kindness and also to fanatical violence and intolerance. It's what the machine does.
And that fundamentally I believe the tragedies the Native peoples suffered were a twin consequence of simple contact on any terms with people from the Old World who bore Old World diseases, and the sad fact that any large movement of those Old Worlders to the New involved greed and ambition that necessarily conflicted with the best interests of the native peoples. Under any religious banner or none, we'd see essentially the same mess, I suppose.
Specifically on slavery--the main restrictions on Europeans enslaving native peoples were that first of all they were terribly vulnerable to the diseases the Old Worlders brought with them; they died out faster than they could be captured and broken. And that escaping slaves would tend to find refuge among peoples they were more or less familiar with, and mass there to strike back. Slaves brought in from Africa had less vulnerability to the diseases and more chancy relations with native peoples they might seek refuge among and were therefore favored. Exploiting native populations worked best when there was a preexisting more or less state-like structure, as in Mexico or Peru, that the Europeans could commandeer and thus extract from a distance as it were, from communities more or less already established. The relations were not much less exploitive, but at least they weren't slavery in the extreme form the Europeans imposed on imported Africans. In places though the Europeans certainly did manage to make slavery of natives pay, or systems practically tantamount to it.
That'd be my thought too. Pagan religions were tolerant of one another, amd while Rome might not last until the founding of the new world, its ideas of assimilation rather than extermination might hold through.
Also: no Christianity likely means no idea of knightly chivalry. And no concept of the three orders (peasants clergy knights). So more merit-based society from an earlier point?
Huh. If you want an idea of meritocracy, Dark Ages style, I suggest you look into Robert "the Devil," Duke of Normandy and father (obviously not by his wife!) of William the Bastard aka "the Conqueror." The Normans were quite brutally practical, and weren't the only feudal lords who were. Basically one was a noble under Robert if Robert found one useful and obedient. He broke up and made marriages to suit his purposes; he pretty much invented the Norman "nobility" from scratch. If he didn't like something a bishop said, he castrated him. I guess it's "merit-based," in a Bioshock sort of Randian sense.


The question is of course, who gets to decide merit?
The notion of the orders might not come wrapped in the pretty paper of claiming it is God's will, but the fact of the orders would be brutally present.
Anyway I'm of the opinion that by that late date or soon after, the Europeans would have come up with some other religion to perfume these stinks with. It's one of the reasons we have religion after all.
As someone else replied to you, pagan religions aren't so much tolerant as oblivious of one another. It was possible for people of different pagan traditions to trade, interact, even rule one another without insisting on uprooting the other's faith of course. But I suggest that the more they interacted, the more problematic it became; rather than generally inspiring religions wars against each other, the general effect was to weaken everyone's attachment to their ancient traditions, which ceased to have the power of relevance to their newly evolving modes of life. This is why the Hellenistic era Hellenizers and the Romans were Seekers, finding little value or meaning in the Olympian traditions once believed by people like Homer and Hesiod.
So they have the virtue of not crusading against each other--but this is because mere contact tended to kill them off as living faiths completely.
...
Yep. No Christendom means no surviving central organization in Western Europe and no Islam, which itself leads to no unifying enemy to Western Europe, and thus there is no idea of Western Europe as a single thing.
Now that's an interesting point! You suggest that even if there were some alternate religion that did play the role of Christianity, and did preserve a substrate of Classical civilization and set norms for the emerging European ones--without Islam as a foil, there still would not be the unifying identity that existed OTL, church or no church.
At any rate it is different without a specific Other to demonize.
Also, Christianity won the Empire because of its message to the oppressed. Less of that would lead to a somewhat more elitist and esoteric public religion mixed with the usual rituals of the lower classes. A more stratified culture. And perhaps a more martial one, considering the leaders of the Roman Empire (and the Germanic tribes that brought it down) got to the top by being good soldiers and not necessarily good civil servants.
Therefore I suspect any successful candidate for an alternate successful public religion would indeed contain a message for the oppressed, one other than "shut up and obey!" At the same time it would also convey that same message too--just as Christianity also did OTL. Any successful universal religion is going to be shot full of ambiguity and paradoxes like that; it serves both master and slave. If the Romans couldn't come up with one then it will be as people mostly read the OP--no public religion at all, to speak of.
I don't think the lack of it would make the oppressors worse. They're as bad as they can get away with being.
Nor would it, Gibbons to the contrary, mean the Empire would be stronger because not undermined by an "effeminizing" religion. My guess as offered above, it would be somewhat weaker due to lack of the extra cohesion the official faith did create. The poor and downtrodden would be about equally bad off, but sooner.
Pagan religions are not necessarily so much tolerant as they are insular. Like China, basically.
I wouldn't call China "pagan" in the original sense though. It has evolved a cocktail of faith traditions--Confucianism, Taoism, and deeply adopted Buddhism--that serve the role of universal religion pretty well. It isn't "unversal
izing" like the Abrahamic faiths to be sure. Chinese culture goes hand in hand with the Chinese state, pretty much; that's part of the mix, with Confucianism stressing the integrity of social hierarchy and importing, or reaffirming, a spiritual aspect to the secular ruler just as all relationships involve a religious obligation to respect hierarchal order.
The faith cocktail has evolved considerably beyond pagan, peasant grass roots, though it remains more rooted to them and contradicts them less than the Abrahamic religions do. It serves the function of a universalist religion for a big empire, though it is not clear to me whether it still lives as a religion or if it will evolve to find its bearings again in a postmodern world where China is merely one of the Great Powers, not the center of the known world. China might of course very well become The Great Power, most likely not by means of external violence but by sheer mass and possibly creative exercise of soft power, and bid fair for the claim to indeed be the world center; whether this will mean a revival of a transformed Chinese spirituality that either undermines or incorporates Maoist Marxism I cannot predict.
It might also become more aggressive under these evolutions.
There was slavery of native Americans.
Rome was actually extremely tolerant of weird customs.They even adopted a lot of them,even foreign gods!
Especially foreign gods, baby! Their own had died on them. They were Seekers. I figure they would have found something eventually, seeking that hard.
"In what direction would Europe have developed if the Roman Empire had not accepted Christianity as its official religion."
Roman culture comes from roman civilization and roman law which predated christianity in OTL.
Notice it does not say no christianity it just says no official religion. You would probably have several christian churches (arian, nestorian, etc) in the empire, a stronger jewish community, along with a Neoplatonic religion and Origenism. Also the academy of athens would be in much better shape.
OP actually says the opposite--not Christianity, and it just seems everyone assumes that implies no official religion at all. The OP did not say that though.
He also did not say no one at all could be Christian, true. But I'd think that if Christianity did exist it
would become the official religion (unless some cult unknown to OTL preempted it); it won the Darwinian marathon OTL and no rival candidate known to history came as close to scratching the itch.
Without some success in satisfying the general social discontent involved in the failure of the old religions (the Romans had already undergone one major transformation under the Republic, replacing in effect their old pantheon with legends of exemplary republican virtues--now that ostensibly secular faith had died on them leaving the pantheon doubly buried) I suspect the secular aspects of Roman/Classical culture would have less resonance than when they were baptized into a faith tradition that survived the collapse of the secular institutions. I certainly think that much of the depth and apparent resonance of Roman lessons in my own education and formed mind probably comes from the relationship between Romans and Gospels as I was raised to hear them. Perhaps my background is not as common as I assume, but as a child most images of Ancient Rome I was presented with were in the context of dramatizations of the Gospels and other Christian stories. Without the living connection of modern society to the state that Jesus lived in, what particular relevance would Rome have that say Persia or Babylon did not? (And indeed we care about them too mainly via the Bible, I'd say). Would we recall the classical Greeks all the more vividly without the fog of Christian doctrine in the way? Or do we in fact remember the Greeks at all because a bunch of monks told us they were important?
Judaea is a unique case because they have only one god and is MUCH resistant towards the empire compared to other provinces.
OK, now I think I'll sketch out where I think we could get Muslims in a timeline where Christianity was somehow or never heard of at all.
Observe here the Arabs, particularly the ones living on the eastern Red Sea coast. Living there more or less, because they are a trading people; the Arabian coast is their home but they wander far from it, many of them do anyway. For thousands of years these Semitic peoples have been living here, with ancient Semitic pagan pantheons they traditionally worshiped.
But they and the Hebrews know each other as relatives; in both Jewish and Arab tradition they are branches of the family of Abraham. This reflects the fact that they are related peoples, by language and some shared traditions. The Jews (for by Roman times so they are properly called, the Judeans of the New Temple returned by the charity of the Persian padishahs to Jerusalem from their Babylonian exile) have come to believe not only that their supreme God is the only one Jews may worship and presumably greater than all other Gods too--they have come to claim nowadays that their God is in fact the only one, all others being some mix of delusion and/or demonic deception. But at the same time, they continue to hold to the ancient covenant they believe in, that they are particularly the people of YHVH, that the relationship of God to Man refers to themselves alone--even that other children of Abraham himself may exist all right, they know they do--but they are not Children of Israel, and the covenant of YHVH and Abraham applies just to them, not to the wrong side of the family.
So do the Arabs just shrug and laugh at the Jewish claim that the Arabs, children of Ishmael, cannot have a relationship with the only God who exists, and go back to happily worshiping their many gods anyway?
The Arabs, as mentioned, live in a sparse land and seek much of their fortunes by trade. They travel far, and meet strange peoples, the Jews among them, but also many others. They are familiar with the debates and discontents about religion that pervade the Mediterranean world and suffer that discontent themselves because they too are now traveling beyond their old pastures and seeing the world turned upside down from the days when their old traditions evolved.
OTL, the Arabs coexisted with the Roman and the various Persian dynasties too. Sometimes one power or the other would claim to rule them--but the grip of those distant emperors and shahs on their harsh desert land was feeble. The Arabs remained not entirely strangers though because they needed to trade and so kept visiting and being visited.
For over six centuries, they were in contact OTL with a Roman Empire in which the religion of Christianity rose from persecuted obscurity to become the state religion of the Empire. Like themselves, it had common roots with the Jewish religion but had become estranged. It too preached one single God for all humanity but unlike the Jewish faith it opened its doors to everyone. It served the role of a universal religion for Rome. One wonders then why the Arabs did not very quickly convert to this faith, perhaps, as many other Christians outside the Empire did, in a version unacceptable to the Emperor in Constantinople but suited to themselves. Some did become Christians, but they remained a small minority. Others sought to refine Arabic paganism along similar lines to Classical Greeks confronting the cruder, earthier paganism of their heritage. But here the matter sat, for six hundred years after the believed time of Christ and many centuries of Christian missionaries visiting them.
And then along comes Mohammed. He tries for a time to get the Arabs accepted by the Jews as their kin and accepted into the Covenant the Jews claimed, but ultimately is rejected as a child of Ishmael. And he turns back to his own people and proclaims that actually, the prophets of the Hebrews were truly prophets to all men, that Abraham and Moses and many prophets before and after them merely spoke the message of how humans were to live in general, but all previous prophets had had their message twisted and corrupted. Now he was once again and for the last, effective time, called to put out the ancient word once and for all, for everyone, for all time. And the Arabs flocked to him, for the most part, and he (eventually) stormed the temples, fortresses and treasuries of the corrupt pagan establishment of Mecca, freed the holy Kabbah that had fallen in ancient times from Heaven, from the true God, and been perverted and prostituted in the name of false lesser gods. And the Arabs burst forth to spread the Truth and conquer the world!
I believe that what Mohammed did was tap into deep roots of Semitic spirituality. The Hebrews showed the way they would tend to evolve once uprooted from the ancient ties to ancient paganism. The prophetic tradition of the Jews was filled with cries for social justice, for righteous behavior in the face of the corruptions of kings and temple priests, for the rights of the poor and the common man. And I suppose the tendency toward monotheism, toward finding a single divine principle and attributing all power, glory, and good to that was rooted there deeply. Mohammed was a man of middling means, who married into modest but honest wealth he helped foster, and he along with many others of his middling trading class hated the extortions and pretensions of the pagan hierarchy at Mecca and other centers. When he found spiritual fire he also found social revolutionary fire; they went hand in hand and seemed to them all to be one and the same. He founded a trader's religion, that reached out to travel and tie together the ends of the Earth. "Travel even unto China in search of knowledge" is a saying attributed to him.
I suggest then that Islam was born out of the social tension of these Semitic peoples drawn into a larger world of trade, and that they rather stubbornly waited to resolve those tensions until one of their own hit upon a new framework that tied together everything they valued into one blinding vision they all could share. With Islam, the Arabs appropriated the covenant of the Hebrews with the Almighty; they appropriated Greek philosophy, and claimed every gift of human culture, which they attributed to the Creator of humankind, to themselves in the name of all people everywhere.
It may be that Christian thought was essential to catalyze this compound, but I find it striking that that spark had failed to strike fire on their apparently dry and ready tinder for half a thousand years. Perhaps conditions had not evolved properly until the sixth century, I suppose--the Arab world of widespread trade had to wait after all until camels were properly domesticated and widespread in Arabia. But I suspect that Islam is a jewel that lay buried in the Arab consciousness, perhaps not fully formed until the right time came, but anyway they would accept nothing but that.
And so--with the example of the Jews before them, and all their cultural historic ties to them (and at the same time dividing them from them) I suspect that that example alone was all it took to turn their minds toward the strict monotheism of Islam, that living in the same situation for the same centuries, they would not require Christian messages to hit upon their own answer that they were waiting for.
If one butterflies away New Temple Judaism, I suppose that might preempt Islam. But once the Jews were allowed to return to Jerusalem, transformed themselves by their contact with Persian Zoroasterism, I think the penny was more or less bound to drop, assuming that Arabia would in the ATL play the same economic role--Christ or no Christ. The Prophet who eventually came might not be named Mohammed, the decade and perhaps century might be different, and the fortunes of the Arabs once they unified themselves and came forth to challenge the northern empires might also be changed, if those empires were less weakened at that point than they were OTL. But even if they do not go on to immediately conquer all of Egypt, Palestine, Syria, Mesopotamia and Persia, I think they'd become a new force here to stay for quite some time, and that they would incrementally eat away at those boundaries, while their new religion would spread not only to Arabs already living outside of Arabia but to similar peoples living similar pastoral/nomadic/trading lives in the Sahara, and perhaps if they could work their way past Persian and/or Roman power, to the peoples of the Central Asian steppes whose lives were also parallel, and win converts there too.
The prophet may not be named Mohammed but the message of submission to God, which is what "Islam" means, would be substantially the same.
If this weren't already going on far too long, I'd sketch out an example of a non-Christian religion that did not even exist in the form I would suggest here, but might in a suitable ATL become a universal religion preempting Christianity (but not Islam!) It belongs, if the author there were inclined to it, in
Dream of the Poison King, because I am talking about a synthesis of Zoroasterianism and Hellenism that could hardly occur in OTL with a strong Roman Empire but might in a world where Mithradates of Pontus manages to hold off the Romans and take Greece under his protection, and his dynasty more or less holds on to Greece and keeps the Romans at bay in the Western Med thereafter.
It would be fun to discourse on this notion of mine as an example of a Great Religion that might be wide and deep enough to pre-empt Christianity (not to mention that the life of Jesus of Nazareth would be displaced quite away in a Pontic ATL) but the OP specified a Roman Empire as OTL and this makes the synthesis really far-fetched instead of plausible. I go on about it in a couple posts in that timeline so anyone interested can look there.
There are several other threads too that would be of interest to anyone who likes this one such as Ernnge's thread that butterflies away Rome completely (and then avoids any pan-Mediterranean empire from ever forming), or that same author's thread on the prospects of the north Germans and Norse resisting Christianization around the Baltic. I should also mention Realm of Millions of Years where the Atenic religion is developed and sustained in Egypt. None of these really point at what alternate cult could have taken Christianity's place in the Roman empire though, nor do I feel up to inventing one.
I haven't done very well at imagining Rome without one; to my mind that's an unlikely historical anomaly and I figure history would heal around the wound, with the Empire falling sooner and harder and perhaps more completely (taking the East down too), a deep Dark Age washes over Europe and out of that Dark Age comes a new "classical" civilization (more advanced to be sure, basically feudal) which will belatedly provide the substrate for a pan-European religion. If the Buddhists or ATL Muslims have not got there first!
