Christianity is an Abrahamic faith related to Judaism and Islam. Founded as an offshoot of Judaism by the followers of Jesus Christ, Christianity was given a firmer organizational structure during late Roman times. Christianity spread from its earliest days mainly via missionary activity (though more heavy-handed pro-Christian rulers did advance/enforce it through conquest as well). Today, it is the world's most followed religion, and has had a huge influence on western civilization overall. As such, the fact that PODs relating to it are popular, should come as no surprise.
This page lists links to discussions about various potential points of divergence in the religion's history - some more plausible, some more far-fetched, but all of them interesting options for alternate developments of Christianity and its interaction with other religions and predominantly non-Christian OTL cultures.