Atheist wants to marry Christian girlfriend. She'll only agree if he finds God.

This is sad. Like on that American Dad episode when Stan find's a "best friend" whom he agrees with everything on except religion, and Stan rejects him due to his own closed-mindedness. I apologize for comparing your situation to a simple comedy cartoon, but it reminded me of it. Anyways the problem is not you, it's her. She needs to (as stupidly idealistic as this sounds) accept you and your beliefs for what they are.

She wants you to "become a Christian", explain to her that if you did become a Christian, it would be a false belief, and you'd be lying to yourself and her. Tell her that you can't just "choose" to believe, because you believe what you believe already.

If she still wants you to become a Christian, then I don't know what to say man. Basically there are two things you can do: Try to reason with her (2nd paragraph), or undermine your beliefs.

I don't think it will work dude. If she wants kids she'll want to raise them to believe in all that stuff and will probably want you to be supportive in that. People that are Christians and are really serious about following the Bible will not marry people who don't share their beliefs. If I can recall from my religious days (I suppose I'm a deist now instead of a Christian) there is some verse about being "not being yoked to non-believers". Hell, when I was serious about Christianity I wouldn't even DATE a girl who I didn't think was devout.