What do you have to be?

[Metonymie]

That's true, of course. But it is not what I was up to.
I mean if you'd claim "I think, there is no god" it would have further implications. If you'd say, for example, "I think there is no god, but I think there is an ultimate truth" (idealistic) you couldn't argue against god, because god is part of an idealistic worldview, too. You could only argue in your idealistic system and propably wouldn't get a result (except, of course, "I believe", "I favour"...)
Therefore, I think, an atheist can't be an idealist and has to be a materialist (If he/she is interested in creating a worldview full of integrity).

That seems completely wrong. Just because God has to be Idealist (i.e., He is immaterial) doesn't mean that Idealists must assent to his existence. That's like saying that because unicorns are material objects, materialists can't object to their existence.

That's not what I meant.
I mean any philosophy with a god in it (like plato's or like a religion), is an idealistic one. You can't argue with plato's theories that the christian god doesn't exist.