Rome is such an interesting counter-point to our present notion of identity. The idea of self-sourcing your own identity was practically blasphemous in the old Republic. A Roman's identity was always Roman and every role was well defined by God and State: you are born as nothing and you are given everything: family, state, role, and your identity changed by the giving. Nero is the earliest Roman I can think of who asserted the priority of his self-identification: "What an artist dies with me." But I think Christianity must have represented a challenge to Roman identity: if you prioritize a personal relationship with one God over Rome, family, position and that God does not prioritize Rome- are you still Roman?
I think you're kind of overthinking this. As far as I can tell the majority of the empire's population (until, at least, the 4th or 5th century) would've consisted of Egyptians, Greek speakers, and Jews (collectively). Most people did not have Roman citizenship until the reforms of Caracalla in the early 3rd century. They most certainly would not be bound by Roman norms and conventions, beyond perhaps veneration of the Roman emperor as required by law.
What allowed Christianity to be successful in the first place, beyond such obvious culprits as a well-maintained road network, relatively safe travel by land, etc, was the religious hyperpluralism that marked the empire. There were SOOOO many fringe cults out there it's not even funny. The religious landscape of the empire was some kind of buyer's market where people just picked and chose what religion they wanted to follow, based on what looked most alluring or whatnot. The reason for this was straightforward: classical paganism was dying to a large degree, especially among Greeks and Romans. The emperor was obviously not a god, and there were many instances where an ambitious general murdered the reigning emperor, then was emperor himself for a couple of months, and then finally was murdered and replaced by somebody else, who himself would likely end up being murdered and replaced.
What people really want is something to believe in. You're never going to be happy wandering from one thing to the next hoping that one day something will stick. Christianity addressed that need better than any other contender.
And that's why I drew the analogy to Rome. In many respects America is not like Rome. We're actually doing a pretty good job at the moment of avoiding many of the key mistakes that proved deadly to them. However, as the Greco-Romans fell into disillusionment with the traditional religion of their day, and then began a desperate search to replace it with something else, so too are Americans, who for the most part have abandoned non-superficial Christianity, dividing themselves up into ideological and identity-based tribes. It is hyperpluralism in our day, like nothing that has been seen on this earth since Rome.
So on its face Social Justice politics, which obviously have come to wholly dominate the Democratic Party in the age of Trump, seem to be a manifestation of chaos as caused by the decline of the traditional Christian faith. But what I might've been missing is this: does it comprise a holistic narrative that the entire population can eventually get behind, and instill a strong sense of moral clarity in a confused people? Can it adequately capture the strengths of religion in its application to contemporary America? What exactly will that look like?
That is to say, is there an order that underlies the appearance of chaos and that will serve the purposes of our generation?