‘The real debate between Calvinists and Open Theists is not so much over the knowledge of God, as both agree that God knows reality perfectly, but over the nature of the reality in which God knows. If the future is predetermine, God foreknows it as certain. If the future has open possibilities given the free will of God, men, and angels, then God foreknows the future as contingent. If the future has some predeterminations and some openness, then God knows the future as having both contingencies and certainties. God knows the certainties as certainties and the contingencies as contingencies. Whatever the nature of reality is, the omniscient mind of God perfectly corresponds to it. A contradiction would exist to say that God eternally foreknows with certainty the outcome of a future contingency, for then God’s mind does not correspond to the nature of reality since He knows a contingency as a certainty.
The Open Theist is consistent in saying that God foreknows contingencies as possibilities, and the Calvinist is consistent in saying that God foreknows predeterminations as certainties, but the classical Arminian is inconsistent in saying that God eternally foreknows with certainty the future outcome of a contingency. The Classical Arminian says God foreknows contingent future events as certainties, though they are free will choices which may or may not be, and yet they are not predetermined as certain by God. The Classical Arminian would be in the dilemma of God foreknowing that which He cannot change, as He eternally foreknows all things but has not predetermined all things. God’s eternal foreknowledge cannot be wrong, so what He foreknows as certain will happen and cannot be otherwise, and yet these events are not caused or determined by God originally.’
source: Jesse Morrell, “Is Open Theism Heretical or Biblical? What is Foreknowledge? Can God Change the Future?” (biblicaltruthresources).