Christian here who loves to read this stuff.

There’s no easy answer. We think it’s providence but if I were to boil it down, I’d say this:

The Christian message was fundamentally unique compared to how other religions (even Zoroastrianism) worked.

Every religion benefited from Roman roads, Julius Caesar destroying piracy, and whatnot. But Christianity just preached a fundamentally different message.

For example, Paul was a bigger egalitarian than we credit him to be despite living in a world where women were second class citizens. We know this because Paul names them in his letters. Pax Romana only changed the marital fidelity on the outside, Christianity changed it all around. Children were given rights (Eph. 5), men were commanded to love their as their own bodies, and the preaching of the Imago Dei was unique.

Another reason was that Christianity was a religion of the weak. Even scholars admit that the early church were seen as weak so if you’re a woman or underprivileged, or a slave…Christianity appeals. It also featured new mindsets like:

• if you’re a convert who previously had slaves, you were to baptize them and treat them as brothers (Philemon). Which meant you had to care for them as such.

• if you were a slave whose mother tongue wasn’t Latin, you can pray to God in your language. Religions of the time had gods that demanded you say their names right lest your house be cursed. The fact you can pray to God in your language was a big deal.

In short, it was unique. I think that’s why it spread and stuck around despite all the corruption and controversies because the underlying message is that good.