This is Wikipedia’s article on Early Christianity. Here’s how the article opens:
Early Christianity is generally considered as Christianity before 325. The New Testament’s Book of Acts and Epistle to the Galatians records that the first Christian community was centered in Jerusalem and its leaders included James, Peter and John.
Of course, if you want to know more about the earliest of early Christianity (by which I mean roughly 30 to 50 CE), check into the writings of Professor Larry W. Hurtado. (You can also search this blog with “hurtado” and find lots of information.)