Peter Cuthbertson has written a post with which I entirely agree.

If your god really is all-powerful, then he’s not exactly going to be harmed by my saying that he doesn’t exist and that you’re an idiot for believing in him. And while my words may sting, you can at least comfort yourself with the knowledge you’re going to heaven and so forth.

I assume the reason some religious people refuse to accept the sentiments above is because such words tempt them into abandoning their silly superstitions and having fun, which would be terrible.