“Atheism” is simply not believing in God. Agnostics, by definition, do not know whether God exists.
Not knowing whether something exists is not a form of “belief.” If you don’t know whether X exists, then certainly you don’t “believe in” X. Every agnostic is an atheist, by definition. There are also additional atheists who are not agnostics.
Atheism is not a “faith,” for the same reasons that “not playing soccer” is not a sport.
