Imagine that there's a meteor heading towards earth. All evidence suggests that it will hit in three days, and wipe out all multicellular life - it's a big one.

What do you do? How do you live your life your last three days?

One school of thought says, disregard the evidence. Assert, against it all, that the meteor is going to miss. Or if that's hard, decide to trust someone who asserts it (this is usually easier).

This is arguably rational. If you're wrong, it won't matter, and you'll probably feel less awful your last three days.

I think creationist's thinking is something like this:

1. If evolution is true, then God is not. 2 Without a creator's purpose, existence is meaningless. 3. If existence is meaningless, you might as well assume it has a meaning, (because that's no less meaningless than anything else!).

To argue against a creationist, it's probably best - or at least, most efficient - to argue against the first stage in that chain of reasoning. If you want to argue against all theists, you can argue against the second.

If you want to argue against all non-nihilists, I guess you can argue against the third, but that is ... literally pointless either way.

So the third point is a good thing to concede in arguments with creationists (and theists, if you insist).