How is it that people who are quite obviously eaten up with Pride can say they believe in God and appear to themselves very religious? I am afraid it means they are worshipping an imaginary God. They theoretically admit themselves to be nothing in the presence of this phantom God, but are really all the time imagining how He approves of them and thinks them far better than ordinary people: that is, they pay a pennyworth of imaginary humility to Him and get out of it a pound's worth of Pride towards their fellow-men. I suppose it was of those people Christ was thinking when He said that some would preach about Him and cast out devils in His name, only to be told at the end of the world that He had never known them. Any any of us may at any moment be in this death-trap. Luckily, we have a test. Whenever we find that our religious life is making us feel that we are good--above all, that we are better than someone else--I think we may be sure that we are being acted on, not by God, but by the devil. The real test of being in the presence of God is, that you either forget about yourself altogether or see yourself as a small, dirty object. It is better to forget about yourself altogether.
~C.S. Lewis, Mere Christianity, (1952)