From Arguing Against Gods:
Another tricky issue is whether or not genuine omniscience is in any way compatible with free will – either ours or the alleged god’s. To start with our free will, it has been observed many times that if a god knows the future with infallible certainty, then what this god knows will necessarily happen – there is no possibility for anything else to occur. We are, then, incapable of altering the future. Although the concept of human “free will” is hotly contested, I’m not aware of any theory of free will which could be considered compatible with a being perfectly knowing the future. If a god knows who will win the next presidential election, then it isn’t possible for anyone else to win. That’s predestination – and some theologians have unflinchingly embraced it, for example John Calvin.