Sometimes we imagine—those of us who try each day to believe—that it must be nice to be an atheist and be so sure. It must be easy, we think with some rue and a little envy, not to have to prove anything because you swear there’s nothing real beyond what you can prove.
C.S. Lewis, the well-known Christian apologist, was baptized in the Church of Ireland, but abandoned his faith in adolescence. He was a professed atheist until the age of 32, when, moved by his friend, J.R.R. Tolkein, he returned to the Church of England.
Knowing that about Lewis helps to fully appreciate these words of his on faith and doubt.
Just as the Christian has his moments when the clamor of this visible and audible world is so persistent, and the whisper of the spiritual world so faint that faith and reason can hardly stick to their guns, so, as I well remember, the atheist also has his moments of shuddering misgiving, of an all but irresistible suspicion that old tales may, after all, be true, that something or someone from outside may at any moment break into his neat, explicable, mechanical universe. Believe in God, and you will have to face hours when it seems obvious that this material world is the only reality; disbelieve in Him, and you must face hours when this material world seems to shout at you that it is not all. No conviction religious or irreligious will, of itself, end once and for all this fifth-columnist in the soul. Only the practice of faith resulting in the habit of faith will gradually do that.
-from “Religion: Reality or Substitute?”
in Christian Reflections, by C. S. Lewis