Atheism as a philosophy and personal life style is the acceptance of the self-assembling universe and the nothingness at the end of life. So on that basis it is not a religion.
However, atheism as practiced by many of its advocates (not all by any means) is a religion because it actively denies the right of other religions to practice. War Memorials having a cross, city seals having a cross, cities having names derived from a belief, all of these have been taken under fire by atheist-espousing organizations. In this case it is a religion.
A belief in a nullity.