Most Catholics who interact with Protestants at all are very familiar with the old saw that the Babylonians or some such ancient people did or believed this, that, or the other, that highly resembles something in Catholicism; therefore (so they contend), the Catholic thing that was prefigured, must be false. Lewis here makes a general observation having to do with Christianity in general, but his brilliant insight applies to these stock anti-Catholic arguments as well:
What light is really thrown on the truth of falsehood of Christian Theology by the occurrence of similar ideas in Pagan religion? . . . Supposing, for purposes of argument, that Christianity is true; then it could avoid all coincidence with other religions only on the supposition that all other religions are one hundred percent erroneous . . . The truth is that the resemblances tell nothing either for or against the truth of Christian Theology. If you start from the assumption that the Theology is false, the resemblances are quite consistent with that assumption. One would expect creatures of the same sort, faced with the same universe, to make the same false guess more than once. But if you start with the assumption that the Theology is true, the resemblances fit in equally well. Theology, while saying that a special illumination has been vouchsafed to Christians and (earlier) to Jews, also says that there is some divine illumination vouchsafed to all men . . . We should, therefore, expect to find in the imagination of great Pagan teachers and myth makers some glimpse of that theme which we believe to be the very plot of the whole cosmic story -- the theme of the incarnation, death, and re-birth. And the difference between the Pagan Christs (Balder, Osiris, etc.) and the Christ Himself is much what we should expect to find. The Pagan stories are all about someone dying and rising, either every year, or else nobody knows where and nobody knows when. The Christian story is about a historical personage, whose execution can be dated pretty accurately, under a named Roman magistrate, and with whom the society that He founded is in a continuous relation down to the present day. It is not the difference between falsehood and truth. It is the difference between a real event on the one hand and dim dreams or premonitions of that same event on the other.
(The Weight of Glory, New York: Macmillan / Collier Books, revised and expanded edition, 1980, edited by Walter Hooper, New York: 83-84, from "Is Theology Poetry?": originally read to the Oxford University Socratic Club on 6 November 1944 and published in The Socratic Digest, vol. 3, 1945)