While there are some religions that are atheistic (certain sects of Buddhism, for example), that does not mean that atheism is a religion. To put it in a more humorous way: If atheism is a religion, then not collecting stamps is a hobby. Despite the fact that atheism is not a religion, atheism is protected by many of the same Constitutional rights that protect religion. That, however, does not mean that atheism is itself a religion, only that our sincerely held (lack of) beliefs are protected in the same way as the religious beliefs of others. Similarly, many interfaith groups will include atheists. This, again, does not mean that atheism is a religious belief.