The 'problem of evil'?

Many atheists will trot out this as the 'killer' blow against Christian theism.

It's not. It's the killer blow against atheism. It's their problem, not my problem!

The typical statement is:

"There's so much evil in the world, I can't believe in/there is no God"

The quick answer: "what's evil?"

[some descriptive answer]

So, you don't like it, but there's no external standard that you can apply...

[some objection]

Without an external moral standard, the definition of 'evil' amounts to mere personal preference. So all you've said is you don't like some things that happen. So what!

And even if you think this is a fatal flaw in Christianity, you still must be able to detect evil...you still must be able to say and conceive of things that 'aren't right' by some external standard. Even to start this conversation, you have to adopt a Christian world view. So, this is your problem, a problem for atheism, not for Christianity.

OTOH, I know that there's 'too much' evil in the world, it is evil because it is inconsistent with the God who is love and made us for relationship with him. There's evil because we are evil! So what are you doing about it your bit of evil? Its action, thought, belief that's missing God.

Why did God allow this to happen?

Because he made us with the ability to make real moral choices, because we are made like him in a way: our thoughts, words, and actions have real significance. They are not random chemical accidents!

Greg Koukl has a couple of great lectures on this: the new atheists, and the problem of evil.

[He made us for relationship with him, so we would enjoy him for ever, but to do this, we had to have true knowledge which meant the freedom to reject that knowledge; a problem? He's solved the problem through Christ bringing us to relationship with God; we've only got to reverse our separation from God and turn to him.]