I don't believe that God exists. I don't doubt God's existence.

One does not start out with the conclusion "God doesn't exist" you begin with the question "does God exist?"
From that doubt you investigate and seek real evidence, when that is not forthcoming (and it never is) then you come to a conclusion that either "God doesn't exist" or "it is probable that God doesn't exist"

You inquiry places the cart before the horse, the effect before the cause, the answer before the question.

Doesn't the breakdown go something like: Agnostics doubt, Atheists reject..? As an atheist, I don’t doubt the existence of any god ... I’m convinced of their nonexistence beyond the shadow of a doubt

It's only a belief

People want all the answers and unfortunately science doesn't have all the answers yet.

???? I'd guess we ALL doubt the existence of a deity....at least, members of Agnostic.com are already ''doubting."

There is always that little voice that says "I could be wrong." There is no evidence for a God or Gods is not the same as there is no God, but for us common folks, it's close enough.