Going by the definition of religion being something like: an organized collection of beliefs that aim to explain the origin of life or the Universe. Then, the various sciences can be said to be religions; perhaps of the atheist variety.
Regardless of your definition, theism is not a religion; Catholicism, for example, is. Atheism is not a religion for the same reason.
Regardless of your definition, theism is not a religion; Catholicism, for example, is. Atheism is not a religion for the same reason.