In the late Roman Empire most people were poor. The state was in the hands of a hugely wealthy elite who called all the shots. The logic for a religion was inevitable. The only source of converts was to appeal to people in poverty. The only source of cash was the government. The winning formula turned out to be highly centralised Christianity. This combined stuff that would appeal to the broke who stood to inherit the Earth if sufficiently meek while guaranteeing that that which was due to Caesar would actually be rendered unto Caesar. Anything that convenient had to be true. It was also worth wiping out any competition. So we ended up with Christian monoculture.
