I see many people on this forum with a one line misunderstanding of atheism. Lets clear this up so we are all on the same playing field.
There is, unfortunately, some disagreement about the definition of atheism. It is interesting to note that most of that disagreement comes from theists — atheists themselves tend to agree on what atheism means. Christians in particular dispute the definition used by atheists and insist that atheism means something very different.
The broader, and more common, understanding of atheism among atheists is quite simply “not believing in any gods.” No claims or denials are made — an atheist is just a person who does not happen to be a theist. Sometimes this broader understanding is called “weak” or “implicit” atheism. Most good, complete dictionaries readily support this.
There also exists a narrower sort of atheism, sometimes called “strong” or “explicit” atheism. With this type, the atheist explicitly denies the existence of any gods — making a strong claim which will deserve support at some point. Some atheists do this and others may do this with regards to certain specific gods but not with others. Thus, a person may lack belief in one god, but deny the existence of another god.
Below are links to a variety of references pages to help understand how atheism is defined and why atheists define it the way they do.
Many have trouble comprehending that “not believing X” (not believe gods exist) doesn’t mean the same as “believing not X” (believe gods do not exist). The placement of the negative is key: the first means not having the mental attitude that proposition X (gods exist) is true, the second means having the mental attitude that proposition X (gods exist) is false. The difference here is between disbelief and denial: the first is disbelief in the broad or narrow sense whereas the second is denial.
The distinction here should be relatively simple and straightforward, but it’s difficult to explain when someone doesn’t automatically “get it.” The stumbling block for many people seems to be the assumption that when faced with any given proposition, the only options are to either believe that it is true or believe that it is false — so when faced with the question of whether any gods exist, a person must believe that either at least one god exists or believe that it is false that any gods exist (in other words, deny that any gods exist).
This is incorrect. It may be that most of the propositions that come immediately to mind are those which we either positively believe are true or deny as false, but there are a myriad of other propositions which don’t fall into either category. A little careful thinking about a couple of hypothetical scenarios may help reveal how this is the case.