People are invariably surprised to hear me say I am both an atheist and an agnostic, as if this somehow weakens my certainty. I usually reply with a question like, Well, are you a Republican or an American? The two words serve different concepts and are not mutually exclusive. Agnosticism addresses knowledge; atheism addresses belief. The agnostic says, I don't have a knowledge that God exists. The atheist says, I don't have a belief that God exists. You can say both things at the same time. Some agnostics are atheistic and some are theistic.
Dan Barker