CS Lewis on Spreading the Gospel

"For my part I believe we ought to work not only at spreading the Gospel (that certainly) but also at a certain preparation for the Gospel. This is necessary to recall many to the law of nature before we talk about God. For Christ promises forgiveness of sins: but what is it that those who, since they do not know the law of nature, do not know that they have sinned? Who will take medicine unless he knows he is in the grip of disease? Moral relativity is the enemy we have to overcome before we tackly Atheism."--CS Lewis in a letter from 1953