Exodus 4:21

And you must not deny free will to Pharaoh just because God says in a number of places, "I have hardened Pharaoh" or "I will harden the heart of Pharaoh," for it does not thereby follow that it was not Pharaoh himself that hardened his own heart. Furthermore, we read that this happened to Pharaoh after the plague of flies had been removed from the Egyptians, as the Scripture testifies: "And Pharaoh's heart was hardened so that neither this time would he let the people go." Thus it was that both God and Pharaoh caused this hardening of the heart: God, by his just judgments, Pharaoh, by his free will.

Now many have been troubled by the story of Pharaoh, in dealing with whom God says several times, "I will harden Pharaoh's heart." For if he is hardened by God and through being hardened sins, he is not himself responsible for the sin; and if this is so, Pharaoh has no free will. And someone will say that in the same way those who are lost have no free will and will not be lost on their own account. Also the saying in Ezekiel, "I will take away their stony hearts and will put in them hearts of flesh, that they may walk in my statutes and keep my judgments," might lead one to suppose that it was God who gave the power to walk in the commandments and to keep the judgments, by his removing the hindrance, the stony heart, and implanting something better, the heart of flesh.

Now let no one along with pagans or Manichaeans dare to censure or blame the justice of God. It is to be believed as most certain that not the violence of God but his own repeated wickedness and indomitable pride in opposition to God's commands caused Pharaoh to become hardened. What does that mean which God said, "I will make him obstinate," except that when my grace is withdrawn from him his own iniquity will harden him? In order that this may be known more clearly, we propose to your charity a comparison with visible things. As often as water is contracted by excessive cold, if the heat of the sun comes upon it, it becomes melted; when the same sun departs the water again becomes hard. Similarly the charity of many men freezes because of the excessive coldness of their sins, and they become as hard as ice; however, when the warmth of divine mercy comes upon them again, they are melted.