I always find it difficult to have a horror story where God gets involved. Because it means either things are going to work out in one form or another, or that the author is going to screw up God. "Black Easter" by James Blish is a rare exception.
Actually to call it a horror story is somewhat misleading. There is no bogeyman jumping out to make you start. No, the horror in this story are in the realm of the ideas. Which to me makes it even worse, but to some folks its rather dry.
The plot has an arms manufacturer getting in contact with a black magician. After he has two people (one the gov of California!) killed by magic, he approaches him with his real goal. He wants the magician to release all the devils in hell for one night just to see what would happen. The magician says sure, it's your dime and he too is fascinated by the concept. Not so fast say the white magicians who are just like the black ones except they only do their magic for "good." Not so fast yourself, says the black magician, you can't touch me, but you are free to watch. So this unlikely band are together as the doors to hell open. What happens next I shall not tell you'd have to read if you dare.
Not really related to the book, but it certainly captures the ickiness