Atheism is not a belief system.
There are, however, systems that support those who are atheist.
"…it is accepted that the Humanist Manifestos are not permanent or authoritative dogmas but are to be subject to ongoing critique."
Agnosticism is a lack of belief. The atheist assertion that no god exists is a belief since it cannot be proven or scientifically tested. Since the humanist manifestos represent groups that may be fairly small or may be large but not all inclusive (of humanists, atheists or agnostics), they are not unlike the tenets of a Christian sect in that those tenets are not all shared by other sects. The way they are clearly unlike the tenets of a Christian sect or other religion is that they are not dogma, are subject to change and no humanist (atheist or agnostic) is expected to accept their precepts.
Though the existence of god cannot be disproved, an understanding of the history of religions and the evolution of the various gods represented by them leads to a clear deduction that the Christian god does not exist as represented by literalists.
That such a god does not exist is not a matter of faith or belief, but of knowledge, understanding and sound reasoning.