The website gives the following definition of "religion."

1.a. Belief in and reverence for a supernatural power or powers regarded as creator and governor of the universe.
1.b. A personal or institutionalized system grounded in such belief and worship.

2.The life or condition of a person in a religious order.

3.A set of beliefs, values, and practices based on the teachings of a spiritual leader.

4.A cause, a principle, or an activity pursued with zeal or conscientious devotion.

Clearly, definitions 1-3 do not apply to atheists since we reject the notions of supernatural powers and spiritual leaders. Definition 4 could possibly apply to atheists, but then, it could also apply to a bowling league or a Britney Spears fan club. The claim that atheism is a religion is generally made by Christians who have been religious all their lives and thus cannot conceive of anyone not having some kind of religion as an integral part of their lives.
It's instructive to point out that theism is not a religion either. Theism simply has to do with believing in a god, which one can very easily do without engaging in any sort of religious activity—to wit, the practice of worshipping that god. A person who believes a deity or higher power exists, but never in his life sees fit to go to a church or pray or partake in any kind of practice designed to worship or revere that deity, would be theistic, but not religious.
Atheism, which is about not believing in god(s), and theism, which is about believing in god(s), are philosophical or theological points of view, but they are not religions.

Posted in: About atheism