This was the one that I have been putting to christians ever since I shook off the yoke of their idiocy in adolescence, except I gave it a moral twist.
Either God can't help out doing good Christian stuff (in which case he's not omnipotent and therefore not God after all), or else he can help but chooses not to, in which case he's a bastard and certainly not worthy of worship. Either way he can fuck off.