He means lack of proof towards theistic beliefs.
If I say i can poop out gold without first consuming it, would you believe me? Probably not, because there is no proof. Therefore, your belief that I cannot shit gold is built upon a lack of proof.
If, however, that didn't apply to you, give me one million dollars, God says so. He also said that asking for any amount of proof will cause you to go to hell for eternity, and that I have 3 days to receive it or you will, again, go to hell for eternity.