Sentence:  <WSD>"Atheism</WSD>, in its basic form, is not a belief; it is the absence of belief." An atheist is not primarily a person who "believes" that a god does "not" exist; rather, he does "not believe" in the existence of a god. 
Meaning of the word marked with <WSD> tags:  Absence of belief that any deities exist (including absence of the concept of deities).