Here's an interesting article on the history of UFOs as vehicles of demons from hell. It also explores various Christian groups and the contradicting beliefs of UFOs as vehicles for angels sent by God to watch over Earth, as well as the other end of the spectrum, being the belief that UFOs simply can't be from other life sources because the Bible doesn't support the existence of life off of this planet. I'm not sure it was exactly what the author was going for, but I think it's an interesting look at certain belief systems and how people have tried to fit UFOs into their preconceived notions.