I have seen variations of this so I am slightly editing it (mostly format and a few grammatical errors) But the gist of it is the same.
There's an old joke about the community-college professor who is militantly atheist. In one class, on a completely different topic, he announces that God doesn't exist, and he can prove it!Heh.TM
The professor shakes his fist in the air and thunders, "God, if you exist at all, I DARE you to strike me down! Strike me down in the next minute! I dare you!"
This annoys several students who are not atheists, among them a recently discharged US Marine, sitting ramrod-straight in the front row. After watching the professor get increasingly smug, he has had enough. He rises from his chair and floors the professor with one punch to the jaw.
The astonished professor exclaims: "What was THAT for?!?"
The Marine quietly responds: "God was busy, so He sent me instead."