If you don't believe in it, then there must be other explanations. What are they?
My own little theory:
At some point pretty soon after it started, christianity developed into the perfect storm.
* It had just the right amount of revolutionary potential, without being an outright call to arms.
* It gave poor people some hope, but also gave the rulers the opportunity to stay in power.
* It merged monotheism and polytheism in a new and exciting way.
* It had a trinity, a saviour, a monotheistic father god and a rudimentary mother goddess, thus diversifying its USP.
* It appealed to those for whom Judaism was an exciting and exotic, but also strange and weird phenomenon.
* It did away with the pesky kosher laws and circumcision, but still demanded a lot from converts, making it appealing to the world-weary, but yet accessible without bloodshed or dieting.
* It had a dramatic origin story, involving murder and betrayal, friendship and hate, death and resurrection.
* It spoke to feelings of guilt and shame and gave an easy solution for those.
* It put faith front and center, rather than race or ritual, making it accessible to everybody and easily transferrable across the globe - while setting sweet honey traps for those inside.
On top of all that, Hera was finally not putting up with Zeus' philandering, and Ares had kinda lost his bow-and-arrows mojo.
As for what happened before that, I don't know. I don't think we can know for sure how it all started. I doubt that it was down to one or two distinct founders, and I think of Acts as a fairytale and of Luke as a big whopping liar, but that's just my evolutionary predilection I'm sure. ;-)