To me, this is key to the story, and explains everything.
Constantine promoted Christianity, and also seized pagan wealth to pay for his imperial projects.
It was a win-win for Christians, with the added bonus of eliminating competing heresies.
It first established patterns of Church-State partnerships which lasted well into the 20th century.
Well, almost correct...Constantine promoted a religious sect that claimed to be Christianity but was the beginning of the Catholic religion...
It was a win-win for the new Catholic religion since Constantine forced his armed forces to get baptized and become Catholics whether they wanted to or not...
This gave the new religion the massive power to overthrow by force those Christians who died holding their scriptures and refusing to bow down to the new state-religion...
And thank God, groups of those Christians and copies of those scriptures perservered thru-out history to this day...