See, "Atheist" comes from Greek. "A-" is a prefix which means "without," and "theos" is a word which means "god." When put together, they mean "without god" - as in, without a belief in god (although, in ancient Greece it was able to be used in reference to anyone with beliefs contrary to the established religions, not just those who did not believe in deities).
Now, "agnostic" also comes from Greek. "A-" we just defined, but the "-gnostic" part comes from the word "gnosis," which means "knowledge." So it literally means "without knowledge," or perhaps "unknowable." Agnosticism is, most often, one of two things: Either a stance that supernatural things are inherently unknowable, and so can not be believed in with certainty
or that one is uncertain of the existence of supernatural things for whatever reason. I.e., they don't know.
By definition, neither have (definite) belief in God (or god or gods).
Now, if you replace the "a-" in "atheist" with "anti-," which means "against," then your statement might be valid. I tend to call myself an antitheist now and then. And don't bring up the notion that the existence of atheism or agnosticism (or antitheism) inherently proves the existence of {G,g}od{ ,s}, because you happen to be an a-flying-purple-space-monkey-ist, but that doesn't make flying purple space monkeys real.
I've heard people say that you can't hate what you don't believe in, but I do hate God while not believing in him/it/them(/her?). It might be argued that I simply hate the idea of him, but I see no difference.
