George H Smith from Atheism: the Case Against God:

The first problem with omniscience is that it cannot be reconciled with any theory of free will in man. If one believes in an omniscient being, one cannot consistently hold that man has volitional control over his actions. If God knows the future with infallible certainty, the future is predetermined, and man is impotent to change it.

Some theologians (such as Calvin) have enthusiastically embraced predestination, but most theologians, sensing the enormous problems entailed by this doctrine, have attempted to defend some theory of volition. Without volition, morality becomes meaningless: we cannot blame or praise a man for an action over which he has no control. Without volition, the Christian scheme of salvation is a farce; men are predestined for either heaven or hell, and they have no voice in the matter. Why does God create men only to save some arbitrarily, and damn others? Why does the Christian bother to proselytize, since men cannot help what they believe anyway? The problems that arise for theology if it affirms predestination are unsolvable, but they necessarily ensue when omniscience is attributed to God.