I believe [in the existence of the devil] not in the sense that it is part of my creed, but in the sense that it is one of my opinions. My religion would not be in ruins if this opinion were shown to be false. Till that happens—and proofs of a negative are hard to come by—I shall retain it. It seems to me to explain a good many facts. It agrees with the plain sense of Scripture, the tradition of Christendom, and the beliefs of most men at most times. And it conflicts with nothing that any of the sciences has shown to be true.
From The Screwtape Letters
Compiled in Words to Live By