"Therefore he has mercy on whom He wills, and whom He wills He hardens." (Rom. 9:18)
"Does not the potter have power over the clay, from the same lump to make one vessel for honor and another for dishonor?" (Rom. 9:21)
"Then I will harden Pharaoh's heart, so that he will pursue them; and I will gain honor over Pharaoh and over all his army, that the Egyptians may know that I am the LORD." (Ex. 14:4)
What is involved in God's act of hardening the hearts of reprobate souls? Is it merely a withdrawal of external worldly constraint, as so many expositors would have us believe? Those who labor diligently to uphold such a position are operating with the long-cherished assumption that God does not create evil or wickedness in the hearts of creatures. So the only way to explain away the biblical assertion that God hardens hearts is to propose that God could not possibly have actually done anything within the heart! Supposedly, the Lord took away restraining circumstances that kept Pharoah's heart "in check."
The 'removal of restraint' interpretation would actually have us believe the opposite of what scripture explicitly teaches regarding God's actions in the Exodus event. The Lord multiplied plagues and misery upon Egypt and Pharoah. The external circumstances sent were those that have caused the humiliation of many otherwise proud men into the dust, fearing even greater retribution and sorrow. But in conjunction with the increasing judgments that God sent upon Pharoah, the Lord correspondingly hardened Pharoah's heart more rigidly against even a nominal repentance that would avoid temporal suffering. The hardening was separate from the sending of terrible calamity. The more suffering that Pharoah endured, the more determined he became to rebel against God in his thoughts and actions.
It is precisely the Lord's constant determination of the condition of each and every heart that is the basis of the foreordaining of evil. If evil is unleashed merely by a removal of restraint, the exact course of that evil is somewhat unpredictable. So the doctrine that God merely permits or allows evil by removing his restraining presence must logically view history as a synthesis of God's predetermination and mere foreknowledge of what will happen (in the case of evil bearing fruit), as does the Arminian. Such a synthesis completely fails to reconcile the paradox that the lack of pre-determination of the precise course of evil, which would be accomplished by controlling the exact sentiments of the hearts of creatures at all times, makes the pre-determination of a desired part of history (the triumph of Grace and salvation) in jeopardy. There can be no reconciliation between a teaching of absolute foreordination and one of partial 'abstract' foreknowledge, ever.
To all eternity, the Lord never abandons his sovereign determination of every sentiment of every creature. He presently sustains Belial in his work of deception, insuring that every desire of the devil's heart will result in the exact course of thought and action that has been determined beforehand. Although the seed of reprobation is present in the non-elect from the very beginning of existence, God directs the sentiments of reprobation in such a manner that there is an increase in wicked desire and action. The seed is planted, watered, grows, and comes to fruition. This is how God hardens hearts, he makes the heart more wicked. As in the case of Pharoah, all such action by God is for the ultimate purpose of bringing glory and pleasure to himself and all his saints!