The really hilarious thing here is that even if everyone agreed that atheism was a belief, all the people who simply lack a belief in god would just make up a new term and start calling themselves "nontheists" or something, and everyone would still have the same beliefs and same arguments. Why are so many people here obsessed with the labels that people use for their beliefs and lack of beliefs? Why not try to address what people actually believe and lack belief in, rather than playing around with the semantics of labels? When someone says that they are an atheist, just ask “Do you mean you are sure that god doesn’t exist, or do you just mean that you aren’t convinced that god exists?” and move on with the discussion from there.
Because theists are constantly using projection to try and portray atheists as members of a rival religion, rather than lacking one entirely. Let the theists define theism, let the atheists define atheism.