A new book is still on its trial and the amateur is not in a position to judge it. It has to be tested against the great body of Christian thought down the ages, and all its hidden implications) often unsuspected by the author himself) have to be brought to light. . . .
Every age has its own outlook. It is specially good at seeing certain truths and specially liable to make certain mistakes. We all, therefore, need the books that will correct the characteristic mistakes of our own period. And that means the old books. All contemporary writers share to some extent the contemporary outlook--even those, like myself, who seem most opposed to it. . . . The only palliative is to keep the clean sea breeze of the centuries blowing through our minds, and this can be done only by reading old books.
People were no cleverer then than they are now; they made as many mistakes as we. But not the same mistakes. They will not flatter us in the errors we are already committing; and their own errors, being now open and palpable, will not endanger us.
Measured against the ages "mere Christianity" turns out to be no insipid interdenominational transparency, but something positive, self-consistent, and inexhaustible. I know it, indeed, to my cost. In the days when I still hated Christianity, I learned to recognise, like some all to familiar smell, that almost unvarying something which met me, now in Puritan Bunyan, now in Anglican Hooker, now in Thomist Dante. . . . It was of course varied; and yet--after all--so unmistakably the same; recognisable, not to be evaded, the odour which is death to us until we allow it to become life.
From C S Lewis' introduction to On the Incarnation by Athanasius. 1046 edition.