I've had several people ask me lately about the difference between an atheist and an agnostic. Best way I can think to explain that is by first defining the words. I've titled this entry "What is Atheism?" because I think any good explanation of atheism necessarily includes both defining agnosticism and distinguishing agnosticism from atheism proper.
To begin, here's an interesting diagram I found on Evolving Thoughts:
Atheist, like agnostic, is a negative. When I say the word atheism is a negative, I mean in the sense of “not X,” or the sense of describing something in terms of what it is not rather than what it is. We use the word “negative” in our regular conversations to imply something bad or wrong. “She has such a negative attitude.” “Don’t be so negative.” This is not the only understanding of the word. “Negative” in this sense has nothing to do with whether something is good or bad. Theologians often employ negatives when describing god, such as eternal (without time), infinite (not finite), and immutable (unchanging). These are not bad attributes; on the contrary, the theologians would not only say these are very good qualities to possess, but would moreover say that such “not X” characteristics are what make god God.
Atheism is a negative. A-theism – a theos (Greek) – simply means “without god.” While one can speak at length about the cultural connotations of the label “atheist,” attaching more meaning to the label is precarious. Not all who adhere to the “atheist” label are the same. Likewise, agnosticism is a negative - a gnōsis - or, "without knowledge." The cultural connotation of the label "agnostic" implies that one is without knowledge vis-à-vis god, gods, or matters of the supernal.
Imagine that I told you there are two men named Bill and Ray standing in an elevator in the Empire State Building. With no evidence to support that claim, you would have no reason to believe it. You also have no need to reject the claim as false either. Certainly there could be two men named Bill and Ray in an elevator in the Empire State Building right now. You simply do not know; appropriately, you should have a lack of belief concerning my truth claim. Many atheists - including myself - have a similar lack of belief concerning a god or gods.
Atheism is not necessarily the denial of god(s), though some atheists do deny god(s). This is called "strong" or "positive" atheism. Positive atheism is the explicit affirmation that god(s) do not exist, whereas negative atheism is a lack of belief in gods (as one might lack belief in the claim that Bill and Ray are standing in an elevator in the Empire State Building), apart from any explict affirmation or belief that god(s) do not exist.
On this note, it is possible - even common - for people to be strong atheists concerning certain god(s) and weak atheists concerning others. For example, Bill the atheist is generally a negative atheist, but a positive atheist when it comes to the gods of Mt. Olympus, the Egyptian gods, and the god of Abraham claimed by Jews, Christians and Muslims. For Bill, the evidence shows that these gods do not exist. Bill does not, however, dismiss the possibility that a god or gods could or might exist. He just has no reason right now to think they do; therefore, he remains a negative atheist with regards to any unknown god(s).
Technically, most theists are positive atheists with regards to other gods. My Christian friends certainly don't believe in Allah or Ganesha or Ra. But the distinction between theist and atheist is simply in whether a person believes in any god. Theists have at least one. Atheists have none.
Agnosticism, on the other hand, is contrasted with gnosticism. Here I'm not referring to the Gnosticism that's the set of religious beliefs and spiritual practices common to early Christianity; rather, I'm referring to the belief that one has knowledge - particularly knowledge of god(s). Gnosticism comes from the Greek γνῶσις - gnōsis - which means knowledge. The distinction between gnostic and agnostic is simply in whether a person believes she has knowledge of any god.
Like atheism, there are "strong" agnostics and "weak" agnostics. A strong agnostic makes the positive claim that we cannot know anything for certain about god(s), including (and especially) whether god(s) exist. A weak agnostic won't make the same epistemological claim, but will merely say that she lacks knowledge. A weak agnostic might even claim to be agnostic about whether anyone can know. In other words, such a person could say, "I don't know whether anyone can know. All I know is that I don't know."
Consider again the chart above. Once the distinctions between atheist and agnostic are made, which necessarily include the distinctions between theist and atheist and gnostic and agnostic, what remains are four categories of believer/non-believer:
Agnostic Atheist: This is someone who both lacks a belief in god(s) and does not claim to know whether there is or is not a god.
Agnostic Theist: This is someone who believes in god(s), but doesn't have knowledge about god(s). Some theists are open about the fact that, while they believe, they can't say they know for certain that the god in whom they believe really exists. For many, this is where faith comes into play. Most theists won't use the agnostic label, even though faith by definition includes believing in spite of one's lack of knowledge (i.e., agnosticism).
Gnostic Atheist: Usually strong atheists fall into this category. A gnostic atheist not only lacks a belief in god(s), she is convinced she knows there is no god(s).
Gnostic Theist: These are the believers in god who will tell you (with great conviction in their voices, usually), that "I know god is real!" They claim to have knowledge regardless of whether they have any reason to believe.
I hope readers find this article helpful.