There can be multiple kinds of atheists. You describe one type of atheist, namely, a strong atheist. The term atheist means exactly what it says: a-theist, without theism. They do not believe in God, or another way of saying it, they lack the belief in God. They don't necessarily believe that God doesn't exist.

Sam pointed out the worthlessness of claiming agnosticism in his Bertrand Russel orbiting teapot argument. Are you agnostic about an orbiting teapot that goes around the sun that can't be viewed with our current instruments? No, it's just an absurd idea to be on the fence about such a claim. There is simply a lack of a belief. We are a-teapotists.

The same goes for God. If there is no reasonable evidence, there is no reason to believe in a God. Sam's main point is that faith is utterly worthless other than to give people bad reasons to do good things where good reasons suffice. Those are almost his words verbatim.

As for who is an agnostic, just about everyone is agnostic. Unless you are talking to an extremely hardcore fundamentalist or an insane person, you can almost always get some to admit that, no, they aren't really sure of anything. Being an agnostic is akin to saying, "Actually, I'm not insane," while good to know, is a trite statement.

As the saying goes, only the madman is absolutely sure, or something to that effect.

-Justin Shroyer