Christ died for our sins. --1 Cor 15:3
In his little essay 'What Is Christianity?' Machen responds to those who say Christianity is a life, not a doctrine.
Of 1 Cor 15:3, he says:
Is it not an account of facts? That is a setting forth of things that happened; it is not an exhortation but a rehearsal of events, a piece of news.
The facts that are rehearsed are not, indeed, bare facts, but facts with the meaning of the facts. 'Christ died' is a fact; but to know merely that never did any good to anyone. . . . Christ died for our sins. . . .
But when you say 'fact with the meaning of the fact,' you have said 'doctrine.' We have already arrived, then, at the answer to our question. Christianity at the beginning, we have discovered, was not a life as distinguished from a doctrine or a life that had doctrine as its changing intellectual expression, but--just the other way around--it was a life founded upon a doctrine.
--"What is Christianity?" in J. Gresham Machen: Selected Short Writings (ed. D. G. Hart; P&R 2004), 95