This reference frame, sort of, sits in the word "atheism" itself. Atheism is defined about theism. It should be the other way round. Let's illustrate this by trying to replace the word "atheism" with a different name, e.g., "freethinker" . If we call all people that are free from religion freethinkers than Christians, Muslims, Buddhists, Hinduists, Jews, Shintoists, etc., are afreethinkers or non-freethinkers.
Get the point? Hope so. But let's take a one more example. Ghosts. I know that many people believe in ghosts, but the vast majority does not . Hence, you would assume that someone you just met somewhere in the street does not believe in ghosts. You might, however, eventually find that this person actually does believe in ghost . Now, the question is: how many people would have to believe in ghosts to swing the balance, so that is you that has to come out as an "aghostic"?
Since repetition seems to be a useful concept in pedagogics, let's add yet another example. Science. If I am supposed to "believe" in the Higgs particle (yes, pun intended), I reasonably ask for the evidence (see, e.g., here for the current status). Similar to so fantastic a thing as gravitational waves . Now, if I tell you there is a teapot in some orbit around the sun, and you do not believe me, how would you feel if I tried to define your non-believing in the existence of this teapot by calling you an ateapotist and claim that your non-belief in the teapot is as good as mine, i.e., it too is "a belief"?
Well, this is the classical burden of proof problem. In the case of the teapot or the ghost, most people will see the point. But when it comes to religion the claim that atheism is a religion too remains widespread.
But it is nevertheless false.
 Religious people do, of course, their best to spread this claim which I see as nothing but cheap propaganda. Guess it helps them to reduce their cognitive dissonances and makes them feel better. A recent example from Sweden is found here. A good comment and some background on this article by Jonna Bornemark are given by Patrik Lindenfors.  I don't think this is a right word in this context and in a way the word "freethinker" contains the same problem as "atheist", but, for the sake of the argument, let's use it here.  Though I might be wrong with my "significant majority" believe here.  And you might be okay with it as long as the person does not try to convince you and does not do you any harm based on this ghost-believing thing.  Wow, look at the list of authors at the end of this paper and compare it with the author list of some "holy" scripture ...