The Bible Dictionary declares: “Latter-day revelation confirms the biblical teaching that the devil is a reality and that he does strive to lead men and women from the work of God.“
In his preface to the 1961 edition of The Screwtape Letters, C.S. Lewis wrote:
The commonest question is whether I “really believe in the Devil.”
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. …God has no opposite. No being could attain a “perfect badness” opposite to the perfect goodness of God….
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 and, as a corollary, to us. These we may call devils. They do not differ in nature from good angels, but their nature is depraved. Devil is the opposite of angel only as Bad Man is the opposite of Good Man. Satan, the leader or dictator of devils, is the opposite, not of God, but of Michael.
What is interesting here is that Lewis’s understanding of devils is so similar to LDS belief and at the same time so different.