The taxonomy I learned classifies someone as an atheist if they BELIEVE there is no god. Those who express doubt as to the existence of a deity are agnostic (unknowing). Atheists can accept this as their only religion, or it can be added as a tenant of faith to Buddhism or Hinduism: "In Western culture, atheists are frequently assumed to be exclusively irreligious or unspiritual. However, atheism also figures in certain religious and spiritual belief systems, such as some forms of Buddhism, that do not advocate belief in gods, and Hinduism, that holds atheism to be valid but difficult to follow spiritually, and does not actively encourage it."
So, I was a philosophy major a long time ago, and wonder if these definitions have moved past me.