Ohhhhh!! now there's a flame toss if ever I've seen one
I would disagree with you. Pax Romana saw Christianity spread far and wide in spite of some long term and major persecution before it was adopted by Constantine a few hundred years after Christ.
It might look different, but even from a sociological point of view, discounting that it's of God and divinely appointed at the time it was; Christianity was known throughout the known world less than 100 years or so after Christ's sacrifice. It was there to stay well before Constantine.