The affirmation of libertarian free will immediately negates the divine determinism that is characteristic of classical theism. But it also (though this is less widely recognized) negates the doctrine of God as timelessly eternal. To assert that humans possess libertarian freedom implies that there is a fundamental, ontological difference between the future and the past. The past consists of events that have already occurred, and are now and forever more unable to be prevented. The future, in contrasts consists in large part of events or better, of the possibilities for events that have not occurred, and may turn out in one way or another. This means that there is a real, objective, difference between past and future, separated as they are by a unique, though ever-changing, present moment. But this claim – that there is a unique moment that is literally and objectively – now stands in deep tension with the view of God as timeless. To see this, it suffices to point out that, if there is a unique present moment, a timeless God cannot know what moment that is. For in order to know which moment is really now, really the present moment, God would have to change, since the truth about which moment is present is itself constantly changing. But change is precisely what a timeless God cannot do. A timeless God cannot know what is happening right now. I believe that almost all theists, once they come to recognize this, will see it as an unacceptable compromise of divine omniscience. A God who has granted true freedom to his creatures must be a temporal God!
This same line of reasoning leads readily to the conclusion that contingent future events – those that are really able to turn out one way or another – cannot be known with certainty even by God. For it is true of God as of human beings that we exist only in the present, not in the future, which does not itself exist. At present, the future is a realm of possibilities for what may or may not come to exist, and is knowable only as such. To be sure, an infinite Mind will know incomparably more about these possibilities, and the likelihood of their being realized, than is knowable to any human mind. And an almighty Being may choose to guarantee that certain events will take place, and in so doing render events certain that would otherwise be mere possibilities. But a Mind that is perfect in knowledge will know all and only that which is inherently knowable, and this means knowing many future events as possibilities and not as guaranteed actualities. That this is so is the most characteristic, and also most controversial, assertion of open theism; it implies that for God, as for us, the future is open, still containing multiple possibilities.