Gnostic - Claim of knowledge
Agnostic - Claim of not knowing
Theist - Believes there is a God
Atheist - Believes there is no God
An atheist believes there is no god or creative force in this reality, they have already made the claim that they reject the idea of an intelligent creative force at work.
A gnostic atheist would says "I know there is no god." An agnostic atheist would say "I don't think there is a god." Both claims take just as much faith as any theist stance.