As a Christian, I was taught that demons were the fallen angels who rebelled against God and were cast out of heaven to Earth. Now they roam around, looking for weak, doubting Christians whom they can trick into not believing in God, or going to the wrong church, or masturbating, or any other sin you can think of. They also convince people that evolution is real and there's no such thing as heaven or hell. Exactly HOW they do all this is never explained, nor is it explained why God had to cast the demons to Earth instead of to some other planet or galaxy where they couldn't hurt humans.