Religions are constantly shifting. As Brakke points out, there was no definitive Christianity when Christianity was still Jewish. Paul never calls himself a Christian, and he was, by his own declaration, Jewish. His interpretation of Jesus varies greatly from that of the eponymous John, of Gospel and Epistle fame. No, there never was a single Christianity. Probably from the very beginning there were Gnostics too. And, again with Brakke, they would have supposed they were following what was to become Christianity as well. Same world, different worldviews. They were not sinister and plotting, any more than other varieties of Christians were sinister and plotting. They were trying to live out lives in accordance with what they thought life was all about.
It has become clear over the last several decades that Christianity never really did unify into a single belief system. Constantine certainly gave it his best shot, but Christianity had spread beyond the bounds of the Roman Empire already, and heterodoxy was rife. No account of Late Antiquity can be honest without noting all the fighting going on among true believers about who was a, well, true believer. Really. Tensions existed early between eastern and western brands of Christianity and schisms became as common as missals. Nobody was really able to put Christianity back together again. In fact, this Humpty Dumpty never was an uncracked egg. I’m afraid I’m taking liberties with Brakke here, but the basic truth remains. Christianity came in its own 57 varieties, most of which didn’t blend very well. The Gnostics come out looking pretty good. That is especially the case when the proto-orthodox start gathering stones. In such a case, it is perhaps time to read the Gnostic scriptures to get a little perspective.