[Originally posted as a comment here]
I think a lot of people misunderstand agnosticism, and therefore atheism.
Agnosticism is the position that something cannot be certainly known, although generally when people speak of being agnostic, they do only mean they are agnostic regarding the existence of god.
In my experience, most “agnostics” are agnostic atheists: they don’t believe in any gods, but they admit that they can’t know that there is no god. You could also be an agnostic theist, and I think many reasonable people who believe in a god accept that they could be wrong about it, but they choose to go with their faith anyway.
I would say that the several types involved in most discussions of the existence of god are as follows:
1) “strong” atheism: I know there is no god
2) “weak” atheism: I believe there is no god
3) “agnostic”: I’m not sure if there’s a god
4) “weak” theist: I believe there is a god
5) “strong” theist: I know there is a god
You can grade these in terms of tolerance, often. 3 would be tolerant of most positions (barring extremism), 2 and 4 allow for some uncertainty and probably tolerate each other best. Using the term loosely, 1 and 5 are “gnostic” positions, and generally not very tolerant of disagreement or waffling.
Of course, plenty of people who agree with 1 and 5 think of it as merely a personal issue, and are perfectly willing to tolerate dissent–much of it is a matter of temperament and social attitudes about tolerance, freedom, etc. On the other hand, the more “aggressive” atheists and religious fundamentalists fall under these categories.
You could class 1-3 as atheist (or at least secular) positions because they don’t promote a god, but because society (in the US at least) is mostly theistic, 3-5 are essentially the status quo–3 only because it doesn’t rock the boat of 4 and 5 too much. Arguably, this is the reason for the term “agnostic” being used as it is–it’s used by people who don’t feel particularly invested in belief/argument for or against god. 3 often sets itself in opposition to 1 and 5: it’s like the ‘opposite’ of extremism.
I would say in attempting to define “atheist”, it’s done either by agreement or opposition, based on one’s agenda.
1-2 will tend to set it up in terms of logic, burden of proof etc, because evidence and reason are typically what matter to people who believe 1-2, and this is extended to the realm of the supernatural.
5 (and at times, 4) will try to frame it as an issue of faith, because they see faith as overriding evidence and logic in importance, at least with regards to a deity.
Atheists are generally trying to get a foothold in a world full of faith and religion using evidence and reason, while theists are generally framing the atheist position in terms of faith and belief, because to do otherwise would be to concede that maybe the atheists are right about the lack of evidence. Essentially, each side is playing a different game, and consequently uses a different definition.
I see it like this: a theist has faith in two sets of knowledge: 1) small f ‘faith’ (confidence) in their interpretation of perceptions and evidence they have encountered and 2) big F ‘Faith’ that the evidence points to god–which is generally where a ‘leap of faith’ is made.
Loosely speaking, (1) relates to how gnostic vs. agnostic your position is, while (2) relates to whether you’re theist or atheist. It’s like confidence in your data vs. confidence in the results you obtained using that data.
To be atheist, you only have to disagree about the meaning or interpretation of the extant data, which means you don’t need big-F-Faith to be an atheist, while you do need it to be a theist.
It’s annoying when religious folk argue that atheism takes faith, because they (at least smarter religious folks) seem to acknowledge that there’s a ‘leap of faith’ that happens when you decide to believe in something that’s not seen. It follows, then, that someone who doesn’t believe in the unseen has made no such leap.
Therefore, in attaining their (un)belief, even the most vehemently avowed atheist hasn’t undergone a similar process of ‘faith’ to even a very level-headed believer in god.