Religious folks tend to use the words “agnostic” and “atheist” somewhat differently than the way that irreligious folks do. This can create misunderstandings, when they and irreligious people try to discuss their theological stances with each other.
These are just generalizations, of course. You can find plenty of people on both sides who use the words in other ways than I describe here, including individual, idiosyncratic ways. However, I see many religious and irreligious people use these words as follows:
Religious people tend to use the word “atheist” to mean someone who feels certain in her / his belief that no gods exist. Related to this, they tend to use the word “agnostic” to mean someone on the fence, unsure whether any gods exist.
Used this way, “atheist” is at one extreme end of the “belief in god/s” spectrum, and “agnostic” is somewhere in the middle of the spectrum. From this view, you’re either one or the other; if you aren’t sure, then you’re not really an atheist.
Irreligious people tend to use the word “atheist” as a straight etymological construction of a-theist — someone without theism, i.e., someone without any beliefs in the existence of any gods. Likewise, irreligious people tend to use the word “agnostic” as a straight etymological construction of a-gnostic — someone without knowledge, i.e., someone who doesn’t know whether any gods exist. (Or, to put it a bit differently, since nobody actually knows with certainty, despite claims to the contrary: someone who recognizes that s/he doesn’t know whether any gods exist.)
Used this way, the two words are on two entirely different spectrums — one about belief (or lack thereof), and the other about knowledge (or lack thereof). From this view, you can be both an agnostic and an atheist. They’re not mutually exclusive. For example, I’m both an agnostic and an atheist: I don’t know for certain whether any gods exist — so I’m an agnostic — and I have no beliefs in any gods — so I’m an atheist. Most irreligious people I know would say they are both.
Some of you might want to interject here about what the dictionary says the words mean. I’m not going to go there. This post is about helping people understand what others are saying to them, not about urging adherence to dictionary definitions.
I must also note here that some folks have trouble distinguishing between “I don’t believe” (i.e., I have an absence of belief in the existence of any gods), and “I disbelieve” (i.e., I have a presence of belief in the nonexistence of gods). They’ll often insist that not believing is really disbelieving. Most self-proclaimed atheists do not actively believe that no gods exist (though many think that the existence of gods is unlikely), but that’s often what religious people think they’re actually saying.
For those who don’t see the difference between “I don’t believe” and “I disbelieve,” imagine we’re talking about items in my refrigerator. Do you, Dear Reader, believe that I have buttermilk in my fridge, right now as you’re reading this? Probably not. I haven’t said that I do and I haven’t said that I don’t. Nor have you looked in my refrigerator. You are simply without the necessary information for a sensible opinion about whether there’s buttermilk in my fridge, and so most of you probably have no belief. Does that lack of belief mean you have an active disbelief, a feeling of certainty that I have no buttermilk in my refrigerator? Again, probably not, because you are still without the necessary information. Just like your lack of belief that I have buttermilk in my refrigerator does not equate to disbelief that I have buttermilk in there, one’s lack of belief in any gods does not necessarily equate to disbelief in all gods.
So when people start using words like “atheist” or “agnostic,” or when they start talking about not believing in God, don’t assume you know what each other means. Just ask. It might lead to better understanding, and more productive conversations.