Technically, we did take the terms "demon" and "angel" from human myth. They line up, more or less, it seemed easier.

[Some people don't care for them, but Michael himself doesn't much mind anymore. There's differences, ones that have become very apparent since an actual Biblical angel and demon appeared here, but the word "demon" still communicates the most important bits.]

Yeah, we have the concept of gods. Humans can't go two centuries without making up a new one.