He defined belief as a conviction. This makes belief non-cognitive like a feeling. Instead, a belief is a cognitive act where we affirm or deny that there is a relationship between a subject and a predicate. In this case we are affirming or denying about what has existed from eternity. Nothing has existed from eternity? All being has existed from eternity? Only some being has existed from eternity?
He says he is certain there is no God in the same way that he is certain there are no leprechauns. This is probably because he has defined God as a magical immortal perhaps like a leprechaun. He approaches the existence of leprechauns and god, as he has defined the term, empirically or through perception rather than by reason and the interpretation of our perceptions.
This approach has not helped us define atheism. It certainly hasn't helped us in figuring out what has existed from eternity and how we can know.