If God is omniscient, this doesn't necessarily mean that he is omnipotent, too. Omniscience is associated with future events that cannot be changed. In similar way, it is impossible to change the causal chain leading up to the emergence of these future events, while omnipotence is related to the power to change and re-arrange this causal chain itself. Now the question is whether God can have it in his power to make such a change and re-arrangement? If yes, then God would contradict himself: how can God foreknow something that will not happen because of an alteration of the causal chain leading to its occurrence at a later time? If not, God would contradict himself, too: how can God be Almighty if he cannot alter what he believed to happen at a earlier time?