Many xians believe in the existence of demons and the devil as much as they believe in their god. Carl Sagan, in his book, The Demon-Haunted World: Science as a Candle in the Dark explains how superstitious humans believe in demons as much as they do their gods:
In Genesis we read of angels who couple with “the daughters of men.” The culture myths of ancient Greece and Rome told of gods appearing to women as bulls or swans or showers of gold and impregnating them. In one early Christian tradition, philosophy derived not of human ingenuity but our of demonic pillow talk – the fallen angels betraying secrets of Heaven to their human consorts. Accounts with similar elements appear in cultures around the world. Parallels to incubi include the Arabian djinn, Greek satyrs, Hindu bhuts, Samoan hotua poro, Celtic dusii, and many others. In an epoch of demon hysteria, it was easy enough to demonize those we feared or hated. So, Merlin was said to have been fathered by an incubus. So were Plato, Alexander the Great, Augustus, and Martin Luther. Occasionally, an entire people were accused by their enemies of having been sired by demons.
So, where did these demons come from? Xians believe their god made everything in the universe. Does this mean that this god made demons? Some people think so. Sagan points out that “in the Talmudic tradition the archetypical succubus was Lilith, whom God made from the dust along with Adam. She was expelled from Eden for insubordination – not to God, but to Adam. Ever since, she spends her nights seducing Adam’s descendents.”
If demons are fallen angels, then why did this god make faulty angels? If he had angels for company in heaven, then why did this god need to create faulty humans? Or is this god just incapable of perfection? Or does he create imperfectly on purpose? Of course, when thinking reasonably, demons are nothing more than figments of human imagination reflecting our own inner fears.