I like to avoid "label words" and just say "I don't believe in God" or even "I don't think God exists". Using label words can make the whole encounter adversarial and make the other less likely to listen to you (or you to listen to them!).

Label words bring to mind identity-based convictions, like "I believe in Allah because I'm a muslim" or "I don't believe in God because I'm an atheist" instead of "I don't believe in God because I've come to exclude the supernatural for such and such reasons...".

This also avoids disputes and confusions over the "correct" definitions of words. For me, saying "I don't believe in God" communicates everything that I need to communicate without having to muddy the waters with the connotations of the words "atheist". Of course the word "believe" is pretty problematic too, but less so.

By the way, it's interesting that the word is "agnostic" instead of "agnostist" or "agnosticist", though it could be just a matter of grammar.

tl;dr - taboo your words often, avoid identity politics