People’s misunderstanding about the nature of atheism can sometimes be traced to a misunderstanding about the nature of disbelief. Comprehensive dictionaries generally define atheism as the “disbelief in or denial of the existence of gods,” and atheists commonly refer to atheism as simply not believing in any gods but is not believing something the same as denying it?

Certainly disbelief and denial aren’t considered the same in the dictionaries, otherwise they wouldn’t need to list both. In the Oxford English Dictionary, for example, the primary definition of “disbelieve” is simply “not believe” and the secondary definition is “reject belief in.” Both of these meanings are distinct from “deny,” which is not listed.

Both of these also match the definition used for “weak atheism,” which is to lack belief in the existence of any gods.
Logically speaking, mere disbelief in the truth of a proposition cannot be treated as equivalent to the belief that the proposition is false and that the opposite is true. If you make a claim and I disbelieve it, I am not necessarily saying that your claim is false. I may not understand it well enough to say one way or the other. Or I may lack enough information to test your claim. Or I may simply not care enough to think about it.

All of these are, of course, possible reactions of an atheist to theistic claims. The atheist may not understand what the theist means by “god” or by certain characteristics of the god (omnipotence, omniscience, etc.). The atheist may not have enough information to determine whether the claim is credible. Or, perhaps, the atheist may find the claim so incredible that it simply isn’t worth thinking about more deeply.

Consider the following analogy: if I tell you that I visited Canada last week, would you believe me? Certainly visiting Canada is common and unremarkable, so there is no reason to think that my statement is inaccurate. However, you also have no reason to think it is true so, although you may accept me at my word, you may just as easily accept my claim as plausible but then not give it further thought because it just isn’t important.

This describes the most basic level of disbelief: you don’t actively believe my claim, but you don’t deny it either. Many atheists take this position with respect to theistic claims when those claims are too vague or incoherent to adequately evaluate. Obviously such claims don’t merit rational belief, but there simply isn’t enough substance to say anything more about them.

We can also go a bit further by modifying my claim to state that I crawled from my house to Canada. Again, such a feat is certainly possible but on the other hand, it also isn’t very likely. Why would anyone do such a thing? While you might step right up to assert that my claim is false, a more likely position would be to “reject belief in” my claim pending further evidence and support. You aren’t actively believing it (because it seems implausible) but you aren’t denying it either (because it’s not impossible).

This is a narrower form of disbelief which is also a common atheistic reaction to theistic claims. In these cases, the claims are coherent and understandable, but there is a lack of substantive support for example, actual evidence to back the claims up. Because the evidence to warrant rational belief is lacking, the atheist does not adopt the belief but the atheist also does not necessarily deny the claim due to a similar lack of contrary evidence. The reaction, then, is to simply “reject belief in” the claim because the theist offers no good reasons to believe.

As we can see, not only are disbelief and denial different things, but there are different levels to disbelief, just as there are to belief. If you are interested in learning in what sense a particular atheist “disbelieves” in a god, you will have to ask. Different atheists disbelieve in different ways and for different reasons.