i don't believe there is a god, so i'm an atheist. when i look at all the available evidence, there's nothing out there that points to anything supernatural.

but i also don't know if there is a god, so i'm agnostic. it's possible that the supernatural exists, i just don't know.

atheism and agnosticism are answers to two different questions. belief and knowledge, respectively.

you can be an agnostic christian, even though most christians are gnostics. you can be a gnostic atheist.

(you can be an atheist christian for that matter, but why would you?)