Exodus 7:3

God constantly says, "I will harden Pharaoh's heart," and gives the reason why he does this. He says, "I will harden Pharaoh's heart and fulfill my signs and my portents in Egypt," as if the hardening of Pharaoh's heart were necessary so that God's signs might be multiplied and fulfilled in Egypt. God makes good use of bad hearts for what he wishes to show to those who are good or those he is going to make good. And the quality of evil in each heart (that is, what sort of heart is disposed to evil) came about through its own evildoing, which grew from the choice of the will. Still, those evils in quality, so that the heart is moved this way or that, when it is moved to evil this way or that way, comes to be by causes by which the soul is driven. And whether these causes either exist or do not exist is not within the power of man. They come from the providence of God that is hidden, most just and clearly most wise, who disposes and administers the universe that he created. So that Pharaoh had such a heart, which was not moved by God's patience to piety but rather to impiety, was the result of his own vice. But that those things happened by which his heart, so evil by its own vice, resisted God's command—it is called "hardened" because it did not bend and agree but resisted unbendingly—was of divine dispensation. It was not unjust to such a heart. It was clearly a just punishment [that] was being prepared, by which those who feared God would be corrected. For example: when money is offered for the commission of homicide, a greedy man is moved in one way, but one who disdains money is moved in another way. The former is moved to commit the crime, the latter to being cautious. Yet the offer of the money itself was not under the control of either of them. Thus motives come to evil men that indeed are not under their control, but they act from these motives as they find them already established from their own past willing. We should consider whether the phrase can be understood in this way: "I shall harden," as if he were saying, "I shall show how hard his heart is."