We, in the Atheist community at least, keep hearing about the "Nones"; those people who profess no religious affiliation. We hear that the percentage of "Nones" is rising. This is a good thing, for us. It sounds good to talk about our numbers increasing. I'm sure, however, that we can really claim all of that.
Some "Nones" aren't really Atheists; but then this gets into the definition of "Atheist". Many people think that "Atheists" are people who believe there is no God. Since it's well-known in the scientific community, however, that we can't prove a negative, then a person who is basing their understanding of the world on science and what can be proven will never say that they "believe there is no God". They will, instead, say, "I believe there isn't sufficient reason to believe in a God." So, the term "Atheist" is, increasingly, becoming defined as, "Lacking a belief in God." This is not the same as "Believing there is no God." A "Theist" believes there is a God. An "Atheist" simply doesn't have that belief. An "Anti-theist" believes there is no God, but that's an extreme view which isn't, and can't be, supported by science.
So, if an "Atheist" lacks a belief in God, then anyone who doesn't have a belief in God is, by definition, an "Atheist". The problem with this, however, is that there are a large number of people who simply don't care! These people generally don't have a belief in God, but they also simply don't care enough about the question to even investigate it or think about it. I know a number of such people personally.
So, can we really claim all "Nones" (people who profess no religious affiliation) as "Atheists"? I don't think so. We need to re-ask the question in a different way. We need to recognize the difference between the "Nones" and the "Don't Cares".