When asked about "his belief in the Devil," Lewis addressed the question in a thought-provoking way in his preface to a revised edition of "Screwtape" in 1960: "Now, if by 'the Devil' you mean a power opposite to God and, like God, self existent from all eternity, the answer is certainly No."
That is, Lewis did not believe in the false theology and caricatures of the devil that have developed over the centuries—whether through art, literature or even today's sports mascots (think Duke and Arizona State).
As Lewis explained, "There is no uncreated being except God. God has no opposite. . . . The proper question is whether I believe in devils. I do. That is to say, I believe in angels, and I believe that some of these, by the abuse of their free will, have become enemies to God. . . . Satan, the leader or dictator of devils, is the opposite, not of God, but of Michael."