The devil Screwtape in "The Screwtape Letters" (C.S. Lewis):

I have been writing hitherto on the assumption that the people in the next pew afford no rational ground for disappointment. Of course, if they do- if the patient knows that the woman with the absurd hat is a fanatical bridgeplayer or the man with squeaky boots a miser and an extortioner- then your task is so much the easier. All you then have to do is to keep out of his mind the question "If I, being what I am, can consider that I am in some sense a Christian, why should the different vices of those people in the next pew prove that their religion is mere hypocrisy and convention?" You may ask whether it is possible to keep such an obvious thought from occurring even to a human mind. It is, Wormwood, it is! Handle him properly and it simply won't come into his head. He has not been anything like long enough with the Enemy to have any real humility yet. What he says, even on his knees, about his own sinfulness is all parrot talk. At bottom, he still believes he has run up a very favourable credit balance in the Enemy's ledger by allowing himself to be converted, and thinks that he is showing great humility and condescension in going to church with these 'smug,' commonplace neighbours at all. Keep him in that state of mind as long as you can."