The proof against the existence of God is in the very claim that he's all-powerful. To be all-powerful, he would have to be omnipotent and omniscient, that is to say, he would know at any given time everything that had happened AND everything that was ever going to happened. But if he knows what's going to happen, that means he is powerless to change the future, and is therefore not omnipotent, because by definition the future fall outside his power. In fact, if he knows everything that's going to happen, he actually has no determinitive power at all.