Where did christianity mainly become established in the first 300 years of its existence?

Christianity started as a small group from the backwaters of the Roman Empire and after two, three centuries go by, lo and behold that same group and its descendants have somehow taken over the Roman Empire and have become the official religion, in fact the only tolerated religion, of the Roman Empire by the end of the 4th century.  

Amuslim warrior especially : one victorious in battle against the opponents of islam —often used as a title of honor.