You needs to take this down a notch from the pure philosophical theory. As an atheist, saying you don't believe in god or that you are sure the He doesn't exist is OK. Even if you couldn't convince Des Carte (he went the other way).

You are sure as you are that celestial teapots or fairies do not exist. Or more to the point, as sure as a Muslim is that Apollo does not drive his chariot across the sky or a Christian is that there are not multiple gods. That is the level where the discussion needs to be conducted.

Not fair to make atheists conduct their analysis at the level where you might be a brain in a vat while eveyone else gets to live in the world where the floor is solid, gravity pulls down & today is followed by tomorrow. The point is to treat the debate just like any other. The existence of god & fertilisation methods for turnips. Use the normal set of tools we use to evaluate everything else.

At the level that you are (rudely) probing atheists, the atheist is agnostic not just about the existence of god, but the existence of dogs, aliens, angels, pyramids & burglars. Certainly agnostic about any of the unseeable uknowable objects of the world. On any level that is meaningful, many skeptical agnostics (after some gentle probing) are atheist.

At least, that's one of my answers on the rare occasion I'm prodded. The other has to do with the lack of usefulness. ;)