What does it mean to be a ‘Christian’? The disciples were first called Christians in Antioch, and from the first, as Paul insisted to the Corinthians, nit was the name of Christ which united them. Simon Magus, who converted to Christianity wanted the charism of healing so much that he offered the Apostles money to receive the gift of the Holy Ghost, for which sin the act of ‘simony’ has been named – that is the act of trying to buy what is holy with money.
Simon was perhaps the first whose case raised the question of what it was to be a Christian. he had been duly baptised by the Apostles and accepted Christ. from the various accounts of him which have survived, he clearly tried to adapt what he knew of the teachings of Jesus to make them fit with pagan philosophies; to some he is the father of gnosticism. Like another early Christian, Cerinthus (who seems to have been of the Judaising tendency), he was thrown out by the Apostles. But it seems unlikely that either man accepted this. There are those who hold that Cerinthus was the origin of the ‘anti-Christ’ in the letters of St John, and his views were rejected by the successors of the Apostles.
There is a common belief that orthodoxy never existed, which often goes along with atheism, but sometimes with various types of sectarian thought, and sometimes with a liberal relativism; after all, if orthodoxy was decided by Constantine or the winning side, you can dismiss it and whatever you decide to believe is fine. Except, as Simon Magus and Cerinthus show, it wasn’t. The same is true for those who argue that the Gospel of Thomas or whatever are as valid as the Canon; no they aren’t, because the early church knew what was and was not Apostolic.
Nothing seems clearer from Scripture itself – from Paul, Jude, Peter and John – than that certain things were not to be believed by Christians. You can’t read the Johannine Epistles, or Peter or Jude and Paul without seeing quite plainly that certain things were not orthodox – that is not right belief. There’s an excellent series by Professor Michael J Kruger here on the Canon, and his whole website should be read by anyone with an interest in any of these things. There is a compelling, clever and excellent review of Kostenberger and Kruger’s The Heresy of Orthodoxy here (which is, by the way, a font of knowledge and good sense on early Christianity), as well as here.
The old trendy view, that there was no such thing as orthodoxy, has been thoroughly undermined by more recent scholarship. Its implication have yet, I think, to be thoroughly digested. Is there, for example, a minimum to which one must sign up to be a Christian? If so, what is it and who gets to define it? That is perhaps one of the reasons why the old belief that there was no orthodoxy gained ground – it avoided dealing with such questions by claiming they were not valid ones.
Many Christians would say that the Nicene Creed is that minimum – it was certainly what the early Church held. I’d be happy with that – but it would be interesting to explore what others thought.