How did Christianity come together as one?

Answer 1

It does not seem as if Christianity ever came together as one. In our very earliest accounts, Paul's epistles, we see that other he called false prophets were teaching a different gospel. We know from the non-canonical Gospel of Thomas that Gnostic Christianity existed in competition with orthodox Christianity from the earliest times. In the middle of the second century, Marcion created his own canon of Christian scriptures, causing other Christians to begin defining what scriptures they would regard as authentic. There have always been competing sects and branches within Christianity.