I'm not here to argue that Atheism is a religion, and I am aware that not all atheists are the same. However, you can't compare organized Atheism to Republicans, Democrats, the NRA or the Mustang club. Those organizations would still exist even if so much as the idea of God did not exist. But if the idea of God did not exist at all, neither would atheists nor organized Atheism. Therefore, Atheism is attached to and dependent on the idea of God. This is why some might think of organized Atheism as a religion.
Also, there are the former atheists turned agnostics, deists and theists who say that they came to a point in their life where denying the existence of God any longer would take more faith than accepting it...their words, not mine.