Well the Akkadian polytheism of Harran was very distinctly focused on the Moon God Sin and the mesopotamian astrological deities in general. In fact the Gods of the classical planets were worshipped widely still in southern Mesopotamia as attested to the fact that corruptions of them remained a consistent part of the folk magic of southern Mesopotamia (and in Mandaeism). Many aspects of Mesopotamian polytheism remind me of Abrahamic religion; the Idea of submission to the Gods who created mankind for the purpose of adoring them and to assist in maintaining order across the world, their devotees were intensely devoted to them but also fearful of divine wrath, and also the Mesopotamian Gods were believed to also control demons and disease and be capable of inflicting harm on humans (This is pretty sharply contrasted with the Neoplatonic view of deities). While archeological evidence of cuneiform past the 2nd century doesn’t exist IIRC (at least what’s in the anglophone record that’s been translated, I read somewhere that there’s lots of cuneiform tablets that never got translated due to political instability but can’t remember where so take that with a grain of salt). There was also substantial Hellenistic influences in Seleucid-Arsacid Mesopotamia that didn’t vanish until the Iranicization of the empire under the Sassanians.
That being said it’s not like Mesopotamian polytheism actually vanished by the POD even when Parthian era temples went into decline personal religion was still very much alive. You said you wanted Buddhism to make inroads into the Near east and influence this neo-Mesopotamian polytheism so here’s my idea; The neo-Mesopotamian religion would primarily focus on the Planetary Gods and would be a mystical fusion of folk religion, Neoplatonism, and south Arab mysticism established by fleeing Sabaean priests (in waves). Reincarnation could be taken from Neoplatonism or Buddhism (or a combo of them) with Nergal being a sort of Yama-Hades type wrathful deity figure who controls the cycle of life. The Gods would be seen as having made a covenant with mankind that needs to be upheld to ensure proper order, and if it’s broken humans would invite themselves to the influence of demons or Chaos of some sort. It would remain a localized religious movement until it manages to convert the leader of an Arab tribe who establishes his own kingdom in Mesopotamia. Kings could be the subject of deification or liberation from Fate at the hands of the Gods who are the masters of destiny. Perhaps the Pandaemonium of this religion could be something influenced by the Judaic presence in Mesopotamia, and demons are seen as born of humans own disobedience of the Gods.
We have to remember also that Christians weren't entirely that friendly to clerical literature that didn't stack up with their beliefs or that they didn't use to promote their ideological aims. There could have been a whole body of literature from the time period that was translated from cuneiform that didn't make it past the filter, since Mesopotamian religion was the religion of Babylon, and we know what Christians and Jews thought of said religion.
Now, I like all of this, but I'm thinking more a religion that syncretizes Mesopotamian religion with the idea of incarnations of Gods that manifest in the form of certain mystic teachers, which Jesus, John the Baptist, Mani, and the Buddha might all be thought to be. This might also be influenced by the Zoroastrian idea of khvarenah. I'm not entirely sure how I'll work it yet, but Jesus never said anything explicitly about the worship of idols, and then of course the Talmudic account of him is wildly different from the Christian one, where he is accused of learning mysteries in Egypt and attempting to induce the Jews to idolatry. So, these Mesopotamians might have a very different interpretation of him, one that works in tandem with the Nestorian idea of separate physical and spiritual personages.
I guess the two ideas don't have to necessarily cancel each other out. I like the idea of it remaining localized until an Arab converts (from Christianity, I think), at which point Arabs might start to try and Mesopotamianize their religion as per popular influence.
I think it’s possible but it’s be more likely imo if some of them remained more cloistered and sedentary in remote regions like the Taurus mountains. But it’s not outside the real of possibility, perhaps some cruel Hunnic overlord has a bone to pick with Christians so he declares them to be unable to own land, turning them into wanderers or semi-nomads.
I think the East will actually turn quicker than the West. Remember that Christianity was a largely urban phenomenon and urban populations have a tendency to shift ideologies quicker than rural ones. Besides, the Huns don't conquer Anatolia, I think. It may be rather that the Goths end up in Anatolia and establish their own state there while the Roman Empire survives in the Levant and North Africa exclusively for a while, perhaps conquered the Red Sea Coast of Arabia. The Taurus Mountains as a hold out may be a good idea here, although Armenia would be even better (historical Armenia, not modern Armenia), especially if it's brought back under the control of the Sassanians during this time. The itinerant populations of Christians would be from Western Europe, and I could actually see policies barring them from purchasing land being enacted before the Hunnic Conquest, especially if Julian reigns a long time and his successors continue his policies and/or decide to mix them with those of Diocletian in terms of the religions pomp surrounding the monarchy. Remember that Christians were originally persecuted because refusing to partake in certain state sponsored cults was seen as treasonous, something like taking a knee during the Pledge of Allegiance. Even the Jews sacrificed to the Emperor, so if the Christians remain unwilling to do so, they could be in trouble with the public, even more so if we imagine a period of crisis leading up to the Hunnic Conquest.
- formal organisation into of mystery religions ie the Eleusinian Mysteries, Samothracian Mysteries, Mithraism, etc church like structures that are state organised and not just located on random islands
- Barbarian invasions of the 5th century as the Roman "Sixteen Kingdoms" period
- East/West split as the South/North split in China
1. This might be doable, I'll have to think about it. I'm not sure that's really how those institutions worked, though.
2. Fair enough. There will be a surviving Roman rump state in North Africa, however.
3. Eventually reuniting. I like this idea as well.