Exactly. IMO, one unified religion can only really come about in western europe when it's under the control of the roman empire. Otherwise, there really is no reason for various independent chieftans and the like to all convert to the same religion.
That's my reasoning as well, but since I am irreligious myself it's possible that I have overlooked some psychological reasons for the success of Christianity.
I guess there were a few alternative scenarios of the division of the Roman Empire after the Crisis of the Third Century. But I think you mean something like the map below attached to this post. In OTL these four parts became hostile to each other there was a war and struggle for domination. But here we assume that successor states became more permanent.
A POD during the Tetrarchy is definitely another possibility. I have some ideas for different events that might be changed in various ways, but the only even remotely firm decision I've made is that the POD should be somewhere between the reigns of Alexander Severus and Constantine.
IIMHO Christianity was just doomed to win on the Roman territories. Of course some Roman successor states would resist Christianity longer than the others - 50-200 years, give or take. But each Roman successor state would have its own version of Christianity more or less hostile to each other.
I'm not entirely sure that I understand your reasoning. Why does Christianity need to be "resisted" like some inevitable flood that can only be stalled, not stopped? For what reason would the non-Christian Romans and barbarians suddenly decide to convert to the worship of Jesus? Mere exposure to the religion is not sufficient, unless Christianity is some kind of memetic disease of unparalleled virulence.
Other than that I agree with you, partly because you seem to know your stuff and partly because you say just what I want to hear.
I see continued disintegration until you have nations based on geographical lines more than anything else. Spain +/- Northern Africa, Gaul to the Rhine and Rhone, Brittania, Italy with surrounding islands, the Balkan Peninsula, Anatolia, and Egypt/Levant. These smaller nations would be easier to manage and would probably survive longer as there would be less chance for massive infighting (it would still happen though). It might cause a delay in the fall of the Roman civilization for another 2-3 centuries though the push of non-Romans peoples will continue into the Balkans and Gaul, perhaps with integration, more likely with warfare. Less is lost and a recovery might be possible earlier, maybe a Renaissance in the 1000s?
That kind of speculation is beyond my capability, but I agree that it's very likely that the successor states would eventually rely on natural borders, if some kind of equilibrium is to be reached.
Though it has already spread to the Atlantic prior to the PoD. By Marcus Aurelius' reign there were already enough Christians in Gaul to "justify" a persecution, and around AD200 Tertullian mentions their existence in Britain and Spain. So they're already everywhere in the Roman world, and all they have to do is keep growing. A supportive Roman State would no doubt help, but they don't need one, and indeed have been managing without one for two centuries.
Why would they grow, though? I have no doubt that many "pagans" in the third and fourth centuries would still convert to Christianity, but there's no reason for the religion to eclipse all others (like it did IOTL) without the support of the state.
If the earlier fall leads to a partial breakdown in east-west communication, that might even help Chritianity in one way. It's most tenacious rival, Manichaenism, is only just starting up in the 3C, so probably won't spread as much or as fast as OTL, where it gave Christianity something of a run for its money in the 4C.
Why would the spread of Manichaeism be retarded?
One important butterfly - this probably means no St Augustine as we know him, since he was a Manichee for much of his life before embracing Christianity, and his early Manichaenism seems to have continued to influence some of his theological ideas even after his conversion.
Well, since he was born decades after my planned POD, he would not even exist, Manichaean or not.
Also, the Pagan sects are still going to suffer. OTL, Constantine seized the treasure from their Temples to put the badly debased Roman currency back on a secure footing. TTL, obviously he won't, but the Barbarian rulers will no doubt plunder it anyway for the usual reasons. So even if Christianity isn't directly strengthened, its opponents are still weakened. And with the Empire gone, it has all the time in the world.
Sure, but it needs more than time to be successful. Jainism has been around for almost three thousand years, and it still has less than five million adherents.
