Confusion of Terms
There seems to be confusion between the meaning of atheism and agnosticism. Agnosticism deals with your knowledge on a subject; atheism describes a lack of belief in deities. These terms are not mutually exclusive.If you don't know whether gods exist, you are agnostic. It simply means "without knowledge."
You can be an agnostic atheist (I don't know, but I don't believe in gods).
You can be an agnostic theist (I don't know, but I do believe in god/s).
You can be a gnostic atheist (I know that god/s do not exits).
A gnostic theist claims, "I believe in god and I know he exists".
For a much deeper analysis of the different belief/epistemological combinations and a very useful graphic representation see tha Free Thinker.