My understanding is that a religion would require an explicitly stated set of beliefs. There is no explicitly stated set of beliefs for someone who calls themself an atheist. However, if someone calls themselves a 1. Materialist 2. Naturalist 3. Existentialist 4. Humanist 5. Secular humanist - it could be argued that these philosophies display an explicit set of beliefs.
Would the evidence of explicitly stated beliefs mean that these philosophies were a religion? For that you would need to be able to demonstrate how a philosophy differs from a religion, or more pertinently, how it doesn't differ. Off the top of my head, I would suggest that a philosophy involves itself with arguments primarily based in reason and that a religion involves itself in arguments primarily based in faith.
Faith, in this context means that regardless of the argumentation or the processes used, that the argument would not under any circumstance change the position of the person of faith. Faith, in this sense, is an unwavering belief, which is not ameliorated or mitigated in the light of new or contradictory evidence or information. Secular humanists, materialists, naturalists or philosophers do not have "faith" of this nature. They may endorse a variety of beliefs, but those beliefs are open to review and to change in light of evidence to the contrary.