Religion says the existence of God can be proved; the agnostic says it can’t be proved; and the atheist claims proof of the nonexistence of God. 
Many members of Alcoholics Anonymous self-identify as agnostics, as opposed to atheists, due to a misunderstanding of the terms. The above passage excerpted from the Twelve Steps and Twelve Traditions (hereafter referred to as the Twelve and Twelve) simply gets the definitions wrong.
I’m not suggesting that the authors were attempting to create a straw man, in order to more effectively argue against atheism.  It’s possible that the authors were merely referring to a commonly used definition of atheism prevalent in the mid-twentieth century. Indeed, many people continue to assume that atheists believe that gods can be proved not to exist (an example of strong atheism). But this is simply not true.
In this brief article, I try to clarify the meaning of the terms atheist and agnostic and argue that most self-proclaimed agnostics are also actually atheists, as the terms are not incompatible.
Do you believe in god(s)?
Imagine you are asked by a believer whether or not you believe in a god. If you respond, as many self-proclaimed agnostics do, with the answer “I don’t know,” you have not answered the question.
The question was not, “Do you know whether or not a god exists?” The question was, “Do you believe that a god exists?” These are two very different questions. One first question addresses what you know. The second addresses what you believe.
The importance of this distinction becomes clear once we investigate atheism and agnosticism a little further.
Atheism and its counterpart, theism, address the issue of belief. For any claim that asserts the existence of a god, a theist accepts (believes) that the claim is true while an atheist is someone who does not. Simply put, an atheist is a person who does not believe the claim “god exists.”
What do you believe about the claim God exists? If your answer is “I believe,” then you are a theist – a “believer.” If your answer is anything else, (I don’t believe or I don’t know what I believe), you are an atheist.
Agnosticism and its opposite, gnosticism, address the issue of knowledge. For any claim regarding the existence of a god, a gnostic is an individual that claims to have knowledge that a claim is true. The agnostic is a person who does not.
What do you know about the claim, god exists? If you do not claim to know whether a god exists, you are an agnostic.
Notice that the terms agnostic and atheist are not mutually exclusive. (To be mutually exclusive means that both are unable to be true at the same time.) Therefore, it is possible to be both agnostic and atheist. Indeed, most atheists are! These atheists, therefore, are agnostic atheists. (Notice also that it is possible to be an agnostic theist. You can believe in god but not claim knowledge of gods existence. Other possibilities include, gnostic theist and gnostic atheist.)
Weak Atheism vs Strong Atheism
Recently, in the interest of promoting clarity of thought, atheists have worked hard to clarify these definitions. Atheists have taken these definitions a step further by delineating between strong atheism and weak atheism.
According to the Iron Chariots Wiki, sponsored by the Atheist Community of Austin, “a ‘weak’ atheist is one who doesn’t claim to know that there is no god, but instead simply lacks belief in a god. This form of atheism is the most common, and is sometimes called ‘agnostic atheism.’”
“A ‘strong atheist,;” on the other hand, “is one who asserts that ‘there is no god.’” These atheists claim to have a rational justification that no god exists. For example, a strong atheist might point out that the Christian God Yahweh possesses characteristics that make his existence a logical impossibility (omniscience, for example).
Although this definition of atheism coheres with the definition found in the Twelve and Twelve, it is important to point out that strong atheists are rare.
For the purposes of this website, AA Atheists, atheism will simply be defined as a “lack of belief in god(s).”
 Alcoholics Anonymous, Twelve Steps and Twelve Traditions (New York: Alcoholics Anonymous World Services, 1998), 28.
 A straw man fallacy is a misrepresentation of the opposing side’s position in such a way as to make the position appear obviously false or even ridiculous. (Please see http://www.ucs.mun.ca/~alatus/2801/StrawMan.html or http://www.nizkor.org/features/fallacies/straw-man.html for more information.)
 The Atheist Community of Austin is doing great work promoting positive atheism through their public access television show, The Atheist Experience (also available on YouTube), their podcasts “Godless Bitches,” and “Non Prophets Radio” and their Wiki, Iron Chariots. Check them out!
 For more information on the logical impossibility of omniscience, please see “The Impossibility of Omniscience” by Souper Genyus, on richarddawkins.net or “The Impossibility of an Omnipotent Omniscient God.”