I don't think the rise of proto-Orthodox Christianity is at all preordained without (or even with) Constantine.
By "Proto-Orthodox", do you mean basically Non-gnostic Christianity? Because prior to the Arian Controversy (which basically takes us up to Constantine's intervention), your basic sects were the Judaizing Christians (who were more or less a "sect" of Judaism), the Docetists (who were more or less the syncretic result of gnosticism and christianity), and the Johnists, those Pauline Christians who were very clear that Jesus was "God made flesh" (though the full implications of that basic understanding would later prove a divisive issue).
Because if so, I think it's pretty safe to say that by 310, the Johnists were the majority of Christians and were where most growth was likely going to be post-legalization regardless of how the Wars of the Tetrarchy played out. Now, does that mean subsequent developments couldn't complicate this? Well...
A move towards de facto monotheism seems to have been underway, but it didn't need to be proto-Orthodoxy. Different Christian sects, Manichaeism, Sol Invictus, mystery religions, Judaism, new Emperor cults, there was plenty of religious diversity and potential for other religions becoming official.
Interesting point about monotheism. I'd still maintain that Sol Invictus had their shot in the 3rd Century, which is passing now; the fact is, by 310, the Roman authorities would just have an easier time uniting the country with Christianity than Sol Invictus, and they knew it. Manicheanism was always going to be too controversial -- ever since Diocletian decided it was a conspiracy by the Persians to destroy Roman-ness, it was going to have an even bigger uphill battle in gaining Roman acceptance than Christians did even at their lowest point, since they first had to convince them they
weren't some kind of Spiritual Fifth Column for the Sasssinids.
And with Judaism -- yes, if the sufficiently powerful Roman authorities were so inclined, they could rebuild the Temple with the hope of undercutting the Christian community; but again, why would they do this? Unless Christianity had already become the major religion in the empire, and they, for whatever reason, wanted to take them back down a peg -- but of course, that's the kind of thing that only even becomes thinkable when the Roman Empire is already unified and the Authorities feel like their actually in a position to dictate Roman Religion to their subjects, rather than needing to work with the religious landscape as it exists to bring about
some kind of civilizational unity. Which brings us to....
The change certainly doesn't need to be completed either, the Western Empire could easily have gone before the transition was completed, leaving a much less Roman structure in many areas.
Actually, that brings us to the interesting scenario I was thinking of -- let's say that the Eastern Empire embraces Christianity (becoming majority Christian by mid 4th Century), while the West, under pagan emperors, continues to defy christianization. Do we really think that, longer term, that Christianity would just stay in the east? I mean, I suppose it's technically possible; OTL saw parts of the western world take centuries to convert, with a lot of gains being lost to pagan invaders, to say nothing of the massive losses one Islam arrived on the scene.
But I don't think that's the more likely scenario -- and to get into why, I have to note that we're now getting to the question I had some hope of getting to -- even though it could only really become an interesting question in the event that “
Christianity still more or less emerges as the leading and/or dominant religion in the West” becomes the broadly accepted position -- which is
how is history changed if Constantine fails to become the Great? That is to say, what if he’s defeated by one of his rivals in the 310’s (preferably by Maxentius, but we can alternately imagine Licinius and Valens beating him a few years later).
Let's look at where the Roman World stood circa 310 CE -- Galerius and his subordinate (and soon to be successor) Licinius are in the strongest position, with Anatolia, Raetia, and everything in between under their belt; to their south and west are allies (or soon-to-be-allies), Maxentius in Italy, and Maximinus Daza in Egypt and Syria (the two would-be-emperors who incidentally would be the last to persecute christians); North Africa was controlled by Domitius Alexander; and in the west, controlling Britain, Gaul, and Hispania (interestingly, the "share" of the empire with some of the fewest Christians), was the son of Constantius, Constantine.
So let's say Constantine's army is defeated, and Maxentius can continue holding Italy, at least for the time being; let's also say Licinius, for his part, has decided to focus for the time being on beating Daza to his south, and securing the east. Who takes Gaul, Spain, and Britain in this scenario? Given that Maxentius's father, the former Western Emperor Maximian, was able to rebel against Constantine for a time around Masilla, I'd say having that city retaken is straightforward enough; from there, Maxentius could project his authority into Gaul, with Britain and Northern Hispanic likely not being too much issue once that's done. Baetica might prove to be a problem, given their sizable Christian community; honestly, the Christians in central Italy will be giving the self-proclaimed Western Emperor enough headaches in that respect. Alternatively, you could see Gaul, Spain, and Britain proving to be too unwieldy for Italia to govern; though in that case, I'd think the Eastern Empire would have even more trouble in that case, with any re-conquest of Italy only doing little to bring the rest of the west into line.
But what about longer term? Well, to get the elephant in the room out of the way -- I think that,
even if there's no full unification of the Roman Empire for at least another century, a Christian East would serve as a powerful source of support for the grassroots growth of Christianity in western provinces. Now, that doesn't mean the future re-conquest would be smooth sailing -- to start, there are those Germans and Goths to worry about, which is only going to become more of a headache starting in the last quarter of the 4th Century when the Huns start pushing people off the Pontic plains.
Been typing for a bit; should look at the latest comment:
for example, It will still take a very long while (not to mention, a lot more defeat and chaos) before people start accepting the partition of the Empire. People will keep attempting it, and the one that briefly manages it will provide a brief window of opportunity for their religion to become the dominant one. Still, the pressures that OTL showcased (economic downturn and rising pressure from more dangerous external groups) remain and are likely to eventually wreck the Empire in ways similar, but not identical, to OTL.
See, the way I see it, the Partition of the Empire has already happened -- nobody prior to Constantine winning the Tetrarchy Wars (with the sole exception of Probus) had managed to solely govern the empire for any meaningful length of time since 249 (which, incidentally, was around the time of Rome's millennial celebrations, 247); and the closest thing to stability Rome knew of for nearly the past century was the period of split rule under Diocletian and Maximian.
The point being,
Constantine was the exception -- he managed to unify the empire under his rule. And even then -- he then split it for a period between his sons, and it was the last one living who managed to unify it again for a time when he went to war against the usurper of his last surviving brother, which is how he was able to leave a united empire for his cousin, Julian. But even then -- when Valentinian pickled up the pieces from Julian's death in Persia and Jovian's quick death thereafter, he took one look at the state the empire was in, and was like "
Yeah, there's no way one man can rule all this", before taking a page from Diocletian's book, and splitting the empire with his brother.
Looked at in that context, I wouldn't be too sure that the unification of the Roman World under a New Augustus is necessarily an inevitability, especially if we're only talking about the Fourth Century.
By 310, I think the old religious status quo was a dead system walking; the Crisis of the Third Century had taken away most of its support, proving the traditional mores as utterly incapable to stop the chaos and leading many to find alternatives - the rise of Neoplatonism is in the same period and it proved a much more resilient rival to Christianity, who ended up incorporating it to a significant degree. I think Christianity was the most likely winner...
This much, absolutely agree with.
...but by all means, there's space for it to lose and even perhaps, with exceptional leadership, for the Imperial cult to do the same incorporation of Neoplatonic thought as part of a restoration. Even with the bold opening declaration, people still had investiment in bits and pieces of the system that could be used to stave off decline and foster innovation:
I'm certainly not going to say this
couldn't happen, but I don't know if it's the more likely scenario than Roman authorities just deciding to let Christianity become popular.
Whoever wins the religious war may not have an Augustine able to help spin this political fall as the ultimate punishment for the old religion's sins, rather than the current favorite one's.
Now this is something I hadn't considered. Might be worth revisiting.
