The narrative we sometimes hear now is "Constantine beat his Roman rivals and became emperor. He was more pro-Christian than his rivals. So this a huge reason why Christianity became big"
But how big was Christianity right before Constantine wins? As in, was its momentum already so great that it would have eventually became dominant without Constantine? How did Christianity get in the running to become a huge religion that an emperor would be willing to throw his weight behind?
It must have already been pretty big for Constantine to decide to throw his weight behind it, right?
I listened to the History of Rome podcast by Mike Duncan. I am trying to fill in the gaps in my mind between these eras:
Christianity looks like a new, small offshoot of Judaism and so Pontius Pīlātus just decides to crucify Jesus
Christianity has underground organizations around Europe
Mike Duncan in his podcast describes Christianity as becoming a religion for poor people and women. And it is seen as a sign of decline of the empire by the aristocracy.
...And then I am trying to get a bigger picture of how we get from 3) to "Christianity has enough popularity/acceptability that a Roman emperor would be amenable to it"
Was the forward momentum towards monotheism and Christianity already inevitable by Constantine's time?
I think it's both--Christianity was already gaining significant traction; and it was greatly assisted with Constantine's choice. I don't think that Christianity would have inevitably become what it became without Constantine; but I also do not believe it would have died off either.
Your point three there regarding the perception of Christianity as a part of the woeful decline of Rome and romanitas by elites is important. For, really the people to whom Christianity appealed the most were indeed the less powerful or powerless, but also they were the majority (taken together). The earliest liberation theology flavor of Christianity appealed to those who had no path to citizenship, or for whom that path would be generations long. And if we think of popularity just as a numbers game, then it's clear that they are the audience you want to capture (not elites whose power and authority cemented snd rarely shared). They were the populace, and the religion was becoming popular among them despite (or, one could argue, because of) its recent persecution.
In an age when what it meant to be Roman (romanitas) was shifting, Constantine chose a theology that would unite his new people--the tall and the small--and make it easier for them to consider themselves an empire. Sure, it didn't work for long and the fracturing of the empire continued until its eventual end; but still it was a pragmatic move on his part to seek a unifier, and to reshape what it meant to be a Roman. Rather than continuing with theologies or religious expressions that were already being judged exclusionary (as seen in the popularity of Christianity), Christianity offered Constantine the chance to be "inclusive" as it were, using a tool that was already circulating among the people. Everyone would be Roman, everyone would be Christian. It would be one ideological empire, and he didn't have to do too much work to make that happen because of Christianity.
That said, I don't think that the power that Christianity amassed over the subsequent 3 or 4 centuries or so would have been possible without that imperial approval at such a crucial time in its development. For Christianity to become a political and social and spiritual force for so much of western Europe by, say, the end of the 6th century; it required the elites to get on board with this new language of authority, and to become experts in the weidling of this new social power. And elites, I as you noted, weren't going to really get on board with this new theology without the push of the most-elite elite in that direction. By the fifth century, you're seeing elites gravitaing towards religious roles as a way to weild power (the rise of the bishop and even the saint); and churches big and small becoming local seats of economic and social power. Monasticism, which is my actual field of study, adds in a layer of spiritual elitism and expertise that cements Christianity's position of power across the empire (fractured as it eventually was). To telescope downward a bit: I don't think you'd have Gregory I as a defacto political leader of the west were it not for Constantine's choice centuries earlier. Constantine's elevation do Christianity made it political, made it politically powerful, and I think made it last.
Tldr: Constantine made a cagey choice, making for a new romanitas that included (not excluded) the bulk of the populace. Christianity, however, needed that approval from the highest echelons of power to become what it became by the high middle ages.