What do you have to be?

[Metonymie]

Ahh
I see what you mean.
But I'd say it depends on how you define "exist".
Plato thought that the material world was fake and that there was a higher level of existence. I don't think that an atheist can believe that and still call himself an atheist with intellectual integrity. He could still not believe in god, of course. But he couldn't come up with a convincing argument for his position.

Uh, why? Atheists don't have to be evidentialists or naturalists. They just have to not believe "At least one god exists". They can believe that you should only believe claims for which there is no evidence; they can be nihilists, and believe nothing. They can believe that the material world is real, but the Astral Plane is even realer, and that's where unicorns and fairies live.

As long as they don't believe in the existence of any gods, they're atheists.

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).