evil

The "Problem of Evil" (wikipedia) has been and is probably the toughest question atheists have posed to Christians. A theist's answer to this problem is called a "theodicy." It takes a good theology to pose a good theodicy, but maybe I'll cover that later.

What I'm primarily interested in right now is not the threat that the Problem of Evil poses for Christianity, but the problem it poses for atheism.

With or without religion, you would have good people doing good things and evil people doing evil things. But for good people to do evil things, that takes religion.
- Steven Weinberg

Well, Steven, there's a problem there.... a fairly obvious problem, once you've looked at it.