I used to know a woman (indirectly) who thought that Satan was her personal tormentor. And even though a few milligrams of haldol was incredibly effect at banishing the evil one, she would always find excuses to go off her meds - I guess so she could do battle with Satan.
I try to be tolerant of most religious beliefs even some of the really stupid fundamentalist shit, but as soon as someone starts talking about Satan like he's a real entity, that ought to be grounds for committal.
Satan doesn't even appear in the bible in his present form as a tempter of souls until the Revelation of John, which was barely allowed into the canon in the 4th century because it is such a late work. But despite barely being mentioned in either the old or new testaments, this cartoon character has a central place in the lives of most fundamentalists. Truly bizarre.