Well, as for how to model the rise of Christianity, it wasn't like there was just a massive influx of believers into the world. The Romans, and indeed many others, actually persecuted Christians for a long time. So as far as the rise of Christianity, perhaps have it start appearing slowly with a lot of hate directed towards it? I don't know which branch of Christianity came about first. My historical focus/specialty is the Roman Kingdom/Republic/Empire. I can look into it for you during my free time, but unfortunately Christmas break is over for me tomorrow, so It might take me a couple days to gather the facts on it for you.
I've done some reading into the spread of Christianity and the offshoots that were present early on, and I think the greatest challenge is working out how to implement it in a way that isn't too railroaded. Bearing in mind the broad range of outcomes possible in CK3, it could very well be that there's games where the Jewish faith doesn't even survive to the approximate date of Jesus' birth, and there might not be a great empire through which Christianity can spread, so its not like we can just have a series of events that are identical each game. In any case, we probably won't look into it further until we expand our map to the Levant.