To confess that God exists,
and at the same time to deny that he has foreknowledge of future things,
the most manifest folly...
For one who is not prescient of all future things is not God.
The Puritan Stephen Charnock, in evaluating the implications of the denial of God's exhaustive foreknowledge of the future free acts of his creatures, and underlining Augustine's remarks, said that in the book of Isaiah "God submits the being of his deity to this trial" and that "If God foreknows not the secret motions of man's will, how can he foretell them? If we strip him of the perfection of prescience, why should we believe a word of Scripture predictions? All the credit of the word of God is torn up by the roots."
In Scripture, from the temptation in the Garden to the temptations in the wilderness, Satan has always sought to cast doubt on the veracity of God's word. The stakes are very high. Open theism gives us a portrait of God that is distorted, misleading, seductive and destructive. It stands opposed to the clear testimony of the Church down through the ages and jeopardizes confidence in God's total truthfulness.
If we cannot trust that God is true to his word, that every word of God will prove true, we cannot trust him at all.