I'd go with "Hey, it's God. I actually AM real, but I'm heading out. You guys are on your own from now on, no more afterlife or anything. So be good to each other! Alright, I'm out. (oh, and just some advice? you might want to get ahead of this climate change thing)"
Yeah, I was hoping the wording of my message would make people give up religious malarkey, but you're probably right. Even being told "don't use religion to be a jerk" wouldn't stop people from doing it.
EDIT: Not to mention, I'd be a little worried about what all those "without the Bible, there's nothing to stop atheists from raping and murdering everyone!" people would do.
Also, if I died and found out religion WAS correct and there was a god, wheeeew doggies I'll bet I'd be in trouble.