No, wait! I've got it! The "one compelling reason for a god"! Free donuts! If you believe in God you go to church, and if you go to church you get free donuts after the service!

Okay, so it's not really so compelling, especially if you're on a diet. How about "If you believe in God you will definitely win the lottery!"? Now that's compelling, huh? Too bad it's a lie.