Every good adventure story needs a hero and a villain. God's the hero, Satan's the villain - they need each other as Superman needs Lex Luthor arch-nemesis and the consummate evil genius. Where would Batman be without the Joker? Would there be a story at all if Captain Ahab had worked for Greenpeace?
It is my firm belief that God and Satan sit down every day and, over a beer, they swap stories, audience figures, tips and tricks, and, of course, what God's son is doing nowadays and how Satan's handmaidens are getting on. Only this makes sense.