I think the matter can be separated into two categories; those who believe in a god or gods, and those who do not. For me, this boils down to two words;
Theism: Belief in a deity.
A-Theism: Without theism.
Belief, and No belief
Within the ranks of believers the strength or conviction of belief can range from weak to strong, but belief in god(s) is a belief in god(s) nevertheless. Likewise with nonbelievers there will be a varying range of conviction, but to not have belief in god(s) is to not have belief in god(s).
A-Gnostic: Without ‘knowledge’ (in this case, of a deity)
To have a believe in a god(s) necessarily implies that there must be some ‘knowledge of god’ going on. If you don’t have any such knowledge then how can you be a believer in god(s)? Looking at it this way indicates clearly to which camp agnosticism belongs. The word ‘agnostic’ is a fairly recent term in the history of humankind, and it can be applied to more situations than just the one about a deity. I am convinced that long before that word came into being there existed people who did not believe in god, and that is another reason why I place primacy on the word atheism, instead of agnosticism. I suppose ‘nonbeliever’, or ‘non-theist’ would be appropriate, but I think atheist works just fine, too.
Exacerbating the situation is that many people do not understand that atheism is a negative statement, not a positive one, meaning that if I tell you I am an atheist all you can actually know about me from this statement is that I do not believe in a god or gods, and absolutely nothing more! On the other hand, a positive statement like “I am a Muslim” is much more telling. For instance you might then ask “Oh, I see you are a Theist and understand you do this or that, etc.).
If someone says they are atheist they are merely telling you what they have no belief in, and you are not therefore entitled to extrapolate any further, saying something like “Oh, I see you are from Cleveland”, or even “Oh, I see you believe evidence exists which disproves god”. It would be premature without further inquiry to assume such things.
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"*Atheism, therefore, is the absence of theistic belief.* One who does not believe in the existence of a god or supernatural being is properly designated as an atheist. Atheism is sometimes defined as "the belief that there is no God of any kind," or the claim that a god cannot exist. While these are categories of atheism, they do not exhaust the meaning of atheism-- and are somewhat misleading with respect to the basic nature of atheism. *Atheism, in its basic form, is not a belief: it is the absence of belief.* An atheist is not primarily a person who *believes* that a god does *not* exist, rather he does *not believe* in the existence of a god."
-George Smith