Can you point me at a pre modern culture that didn't have religion? I don't know any off hand.
And this goes back to what I was saying at the start of this thread: the church community is nice. But it is tied to an insistence in belief. Like, you have to have these ideas in your head. That's actually kind of f*cked up if you think about it, and kind of an aberration in the context of spiritual practices in all of human history. Most of the time it was: put this idol in your house or give a sacrifice for a good harvest and we're good. Nobody cared if you believed it or not. It's a series of practices to hopefully alter the world. And some good stories to go with it.
The Romans were incredulous at the early Christians not because of what they believed ("sure, Christ, why not...") but because they refused to follow along in community practices (no sacrifices to the emperor or city gods) and insisted that _only_ their beliefs were correct.
From Merriam-Webster: https://www.merriam-webster.com/dictionary/religion
2 : a personal set or institutionalized system of religious attitudes, beliefs, and practices 4 : a cause, principle, or system of beliefs held to with ardor and faith
I think that applies universally or very nearly so to human societies - which is amazing because very little is so universally human.
> many belief systems wouldn't fit what we call religion now
I see that as just a blind spot caused by our Judaeo-Christian heritage, not a fundamental issue.
You've got some interesting points though about how Christianity differs from pagan religions (actually I think it applies to Judaism and Islam as well.)
Further, Christianity was the first "religion" to insist on its exclusivity. Others were either "people" religions ("Jews believe this"), animistic/traditional spiritualities, or highly polytheistic systems that willingly absorbed or included other gods / belief systems easily.
Christians were the first to come along and say: you [anyone in humanity] will burn eternally if you don't practice this belief exclusively, only we are correct. And not only that, by the 3rd century they were executing each other for disagreements about very arcane subjects (consubstantiality, etc.) And then later killing non-Christians as well.