The differences between atheism, theism, and agnosticism always seemed pretty clear to me...ask each of these individuals the question "Is there a god?" and they'll say either "No", "Yes", or "I don't know". It seems irrational to say that you do or don't believe in a god if you also claim that its impossible to know if one exists.
Examples are fun.
I don't know what color Martian rock is 30 miles beneath the surface. The way you (any many people in common usage) look at atheism/agnosticism/theism would be asking "hey, what color is Martian rock 30 miles down?" and me having the option to respond "yes/no/I don't know" (and, of course, choosing the latter--agnostic but neither atheist nor theist).
The perhaps more technical use of agnosticism (and what Huxley actually meant by the word) and of atheism/theism gets more complicated. Here atheism and theism aren't a stance, but a lack or presence of a specific belief. So you ask, "Rhizomatic, do you have the belief that Martian rock is green 30 miles beneath the surface?"
Here "I don't know" isn't an answer; I either have the belief or I don't. I have no way of knowing the truth, and I know that it is possible that the rock is green, though it's also possible that it's a different color. I'm agnostic in the sense that I know that I can't know the answer (we'll pretend that there is next to no scientific knowledge of Mars and its composition for me to look up). While I don't actively reject the beleif in green Martian rock I also don't posses it.
Everyone
has to have or lack belief in god(s). Those are the
only two options, and (a)theism determines where you fall on that scale. If you don't know if there is a god or not, then you obviously don't possess belief in god (theism), and are thus a (weak) atheist. It seems obvious that humans cannot prove or disprove with certainty proposed entity that infinitely transcends them and cannot be directly observed at will, and I can't really say that I see any contradiction between that epistemological stance and holding or lacking belief.