Christianity holds that God (a) has free will and (b) is omniscient. For God to have free will, he must be able to choose at any moment in time ("the present") between two options. For God to be omniscient, he must know everything, both in the present and in the future. It follows that at any moment in time ("the present"), when faced with several options, he will know which one he will pick. Thus, since God has no choice in his decisions, he cannot both have free will and be omniscient, which proves that the Christian ideal of God cannot exist.
One counter-argument maintains that the argument assumes God would exist within the constraints of time, which has no basis in traditional Christian beliefs. Since time is a property of or part of the Universe, and God is separate and distinct from the Universe he causes to exist, God therefore exists outside of or beyond time; and so there is no contradiction between his omniscience and his free will. The argument should have been presented not in terms of time but in terms of causality, for time could not be discerned in a causal system at equilibrium. Whatever is not causal is random, so omniscience cannot extend beyond a causal network. For God to be omniscient, It must know the entire causal network. Therefore either there can be no options or God cannot know which path is selected, which proves the Christian ideal of God cannot exist.
A version of the freewill argument has been used by some theologians to attempt to prove that God's omniscience and Man's free will are incompatible. For if God knows what we are to choose before we choose it, how are we free to choose otherwise? Hence, either God is omniscient and we are determined or God is not all knowing and we are free to make our own choices. This is one of the essential arguments behind the doctrine of predestination.
See atheism, Arguments against the existence of God, secularism, rationalism, agnosticism.