In an article entitled 'Is There A God?' (never published, 1952) the famous philosopher Bertrand Russell wrote:

"If I were to suggest that between the Earth and Mars there is a china teapot revolving about the sun in an elliptical orbit, nobody would be able to disprove my assertion provided I were careful to add that the teapot is too small to be revealed even by our most powerful telescopes. But if I were to go on to say that, since my assertion cannot be disproved, it is an intolerable presumption on the part of human reason to doubt it, I should rightly be thought to be talking nonsense. If, however, the existence of such a teapot were affirmed in ancient books, taught as the sacred truth every Sunday, and instilled into the minds of children at school, hesitation to believe in its existence would become a mark of eccentricity and entitle the doubter to the attentions of the psychiatrist in an enlightened age or of the Inquisitor in an earlier time."

Many people mistakenly believe that the atheist position is one of certitude about the non-existence of a god, but this is not necessarily the case. I have heard it said that agnosticism is the rational position to take on the existence of a god and that atheism is as dogmatic a position to take as theism, but I think that is a misunderstanding of these terms. Agnosticism deals with claims that we are unable to make definitive judgments on because of a lack of evidence, just as in the case of the celestial teapot. However, agnosticism does not address belief, only knowledge, and could apply to a wide array of claims besides religious ones. In fact, you could be an agnostic and still believe in a god (agnostic theist); so, stating that you are an agnostic doesn't say anything about what you believe. In regard to the question of a personal god, there are really only two categories; you are either a theist (believe in god) or an atheist (don't believe) - there is no middle ground.

Atheism is defined as: "Absense of, or rejection of, belief in the existence of god or gods" (Wiktionary). There are two main categories of atheism: agnostic atheism and gnostic atheism; also referred to as weak atheism and strong atheism, negative atheism and positive atheism, implicit atheism and explicit atheism. Gnostic atheists claim that they know that a god does not exist. Agnostic atheists don't claim to know that a god does not exist but maintain that there is no evidence that one does exist. However, there are certain descriptions of god that one might take the position of a gnostic atheist, and on another, take the position of an agnostic atheist. It all depends on how the god in question is defined.

If someone is uncertain about whether or not they are an atheist, they can employ this very simple test: if you can't make the statement "I believe in a god" you are an atheist. It all hinges on the word believe; which is defined as: "To accept as true" (Wiktionary). If you are doubtful, uncertain or skeptical, but not necessarily convinced of the non-existence of a god, this still means that you haven't "accepted it as true" that one does exist. In other words, you have an "absence of belief" in a god - which is the definition of atheism.