Atheism: I believe and know
Hopefully that conjunction "and" there indicates the presence of two rather different strains of atheism rather than being a gross mischaracterization about atheists as if all atheist make a positive claim about knowledge.
Once more for effect: as an atheist I can only tell you what information has been made available to me to confirm the existence of "God" and so far that information has proven to be insufficient for me to believe in "God". This is 100% NOT the same as "knowing" there is no God. Simply failing to believe in God until sufficient information comes to overcome that position.