The beginnings of Christianity constitute a fairly definite historical phenomenon. The Christian movement originated a few days after the death of Jesus of Nazareth. It is doubtful whether anything that preceded the death of Jesus can be called Christianity. At any rate, if Christianity existed before that event, it was Christianity only in a preliminary stage. The originated after the death of Jesus, and the thing itself was also something new. Evidently there was an important new beginning among the disciples of Jesus in Jerusalem after the crucifixion. At that time is to be placed the beginning of the remarkable movement which spread out from Jerusalem into the gentile world–the movement which is call Christianity.
About the early stages of this movement definite historical information has been preserved in the Epistles of Paul, which are regarded by all serious historians as genuine products of the first Christian generation. The writer of the Epistles had been in direct communication with those intimate friends of Jesus who had begun the Christian movement in Jerusalem, and in the Epistle he mike it abundantly plain what the fundamental character of the movement was.
But if any one fact is clear, on the basis of this evidence, it is that the Christian movement at its inception was not just a way of life in the modern sense, but a way of life founded upon a message. It was based, not upon mere feeling, not upon a mere program of work, but upon an account of facts. In other words it was based upon doctrine.
–J. Gresham Machen, Christianity and Liberalism (Grand Rapids, MI: Wm. B. Eerdmans, original 1923, republished 2009), 18