I’m confused, the biblical canon was established in the late 300s, over a thousand years before modernism or the printing press.

I should clarify: there was a "canon", but every church basically just did its own thing. They all had their own relics and traditions and special sermons passed down, completely different from every other church. There was a unifying holy world of the Catholic canon, but every fiefdom had its own relics and traditions that it adhered to on top of that. That's what pissed Martin Luther off so much, all of this extra fanon that was made up and tacked onto the canon. Folk mysticism was tolerated by the Catholic Church as long as they payed their dues.

When the reformation came and zealots started burning reliquaries and melting down shrines, that was when Christian spirituality became religion.