the burden of proof is on whomever makes the extraordinary claim
Christianity and it's attendant liturgical documents and period artifacts and accumulated oral history are the proof.
Why were some of these writings accepted and some weren’t?
Ask the folks who made the decision, assuming you believe the Council of Nicea was real.
I’m not addressing Christians at all
¯\_(ツ)_/¯
You're not concerned with evidence if you're reflexively denying it.