But, it will be said, Christianity is a life, not a doctrine. This assertion is often made, and it has the appearance of godliness. But it is radically false, and to detect its falsity one does not even need to be a Christian. For to say that ‘Christianity is a life’ is to make an assertion in the sphere of history.
. . . . . . . . . .
About the early stages of this movement [that is, Christianity] 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 Epistles he makes 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 (Eerdmans, 1923), 19, 21.