How many honest to pete monotheists are there? Christianists and Muslimoids all believe in Satan, and in angels and jdinns and demons. Jews mostly credit Satan too, and those who don't still set a place for Gabriel -- or whoever, I don't have my notes here -- at Passover. All those b'lievers may class their Yehooavow as the one true only deity, but, standing off at this distance, I merely note their folklore, and I certainly don't accept it.
Other cults teem with gods n demons n sperrits n whatall. Just favoring one over the others and praying to him on a brick is hardly exclusive monotheism.
Over the millenia, gods rise and fall, often turning into, yessir, demons, angels, spirits, and the habit of nailing a horseshoe over the door.
For decades now, we've seen fools trying to revive (or reinvent?) defunct gods, with some success. I'm inclined to think that pure-quill monotheism can never satisfy anybody's religious cravings, because it's so goddamn boring. It explains everything with the ringing of one dull bell: GODDIDIT. Even so-called simple folk can't stay satisfied with that. Our minds swarm with questions, fantasies, and conjectures. We want the supernatural to be at least as varied -- and as strangely beautiful -- as the world we inhabit.
But, and this is what may eventually save us from religion, as we learn more about the world and the universe, the more satisfying they become because we know so little but now we can find out. After all, those innumerable supernatural explanations were never really adequate, hence their continual proliferation.