Stating I don't know is what atheism is. It is no different than aclausism. I can't prove that Santa Claus doesn't exist somewhere out there. But the burden of proof is not on the negative claim "Santa doesn't exist" rather it is on the assertion that something or someone exists "Santa exists".
In order for the hypothesis of Gods to hold up falsifiable evidence must be provided by the one postulating such. One has to start from atheism for an evidence based theorem to hold up. If one starts from a belief in clausism/theism the results are more often than not tainted by bias.