[CCIP]

To weigh in as a linguist: the "a" in "atheist" and "agnostic" very specifically means "against". An atheist is indeed someone who is very specifically against the idea of god. An agnostic is someone who denies the possibility of knowing (gnosis). There's nothing tentative about the "a". It's sort of like the difference between "immoral" and "amoral". The latter very much indicates a conscious denial, not a passive lack.

The whole picture is pretty complicated by the way. There are definitely agnostics, and there are also theistic and non-theistic believers who would disagree with religious followers, atheists and agnostics equally. I think a lot of people use "atheist vs. believer" or "atheist vs. agnostic" binaries in a way that really only serves to further their own cause, whereas the reality is a lot more diverse.