Men say, "How are we to act, what are we to teach our children, now that we are no longer Christians?" You see, gentlemen, how I would answer that question. You are deceived in thinking that the morality of your father was based on Christianity. On the contrary, Christianity presupposed it. That morality stands exactly where it did; its basis has not been withdrawn, for, in a sense, it never had a basis. The ultimate ethical injunctions have always been premises, never conclusions. Kant was perfectly right on that point at least, the imperative is categorical. Unless the ethical is assumed from the outset, no argument will bring you to it.

source: C. S. Lewis (1898-1963), "On Ethics" tags: Morality, Ethics