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. […]

As the expression is somewhat harsh, many commentators, as I have before said, take pains to soften it. Hence it is that some take the words in connection, "I will harden Pharaoh's heart by multiplying my signs;" as if God were pointing out the external cause of his obstinacy.

And I will harden Pharaoh's heart,.... See Gill on Exo 4:21. and multiply my signs and my wonders in the land of Egypt; work one miracle and wonderful sign after another, until they are all wrought intended to be wrought; and which he had given Moses power to do, and until the end should be answered and obtained, the letting go of the children of Israel.

The dispute between Moses and Pharaoh begins, and a famous trial of skill it was. Moses, in God's name, demands Israel's release; The contest is between the power of the great God and the power of a proud prince; and it will be found, in the issue, that when God judgeth he will overcome.

I will harden Pharaoh's heart--This would be the result. But the divine message would be the occasion, not the cause of the king's impenitent obduracy. I may lay mine hand upon Egypt, &c.--The succession of terrible judgments with which the country was about to be scourged would fully demonstrate the supremacy of Israel's God.