You always capitalize atheist. Why? Even the atheist blogs do not do so.
I take my lead from the Religious Tolerance website:
We recognize that many Atheists reject the concepts that Atheism is a religion, or is similar to religion, or functions in some ways as a religion. However, we feel that it is appropriate to capitalize these terms as we have capitalized the names of organized religions.
I do it out of respect. If “Atheist” is not a proper pronoun, it should be. (I do not change it when I’m quoting; nor to I sic it).