Robert Flint

...properly a theory about knowledge, not about religion. A theist and a Christian may be an agnostic; an atheist may not be an agnostic. An atheist may deny that there is God, and in this case his atheism is dogmatic and not agnostic. Or he may refuse to acknowledge that there is a God simply on the ground that he perceives no evidence for his existence and finds the arguments which have been advanced in proof of it invalid. In this case his atheism is critical, not agnostic. The atheist may be, and not infrequently is, an agnostic.