1 “But,” say they, “God hardened the heart of Pharaoh and of his servants.” Those, then, who allege such difficulties, do not read in the Gospel that passage where the Lord replied to the disciples, when they asked Him, “Why do You speak unto them in parables?”— “Because it is given unto you to know the mystery of the kingdom of heaven; but to them I speak in parables, that seeing they may not see, and hearing they may not hear, understanding they may not understand; in order that the prophecy of Isaiah regarding them may be fulfilled, saying, Make the heart of this people gross and make their ears dull, and blind their eyes. But blessed are your eyes, which see the things that you see; and your ears, which hear what you hear.” For one and the same God [that blesses others] inflicts blindness upon those who do not believe, but who set Him at naught; just as the sun, which is a creature of His, [acts with regard] to those who, by reason of any weakness of the eyes cannot behold his light; but to those who believe in Him and follow Him, He grants a fuller and greater illumination of mind. In accordance with this word, therefore, does the apostle say, in the Second [Epistle] to the Corinthians: “In whom the this world has blinded the minds of them that believe not, lest the light of the glorious Gospel of Christ should shine [unto them].” And again, in that to the Romans: “And as they did not think fit to have God in their knowledge, God gave them up to a reprobate mind, to do those things that are not convenient.” Speaking of antichrist, too, he says clearly in the Second to the Thessalonians: “And for this cause God shall send them the working of error, that they should believe a lie; that they all might be judged who believed not the truth, but consented to iniquity.”
2. If, therefore, in the present time also, God, knowing the number of those who will not believe, since He foreknows all things, has given them over to unbelief, and turned away His face from men of this stamp, leaving them in the darkness which they have themselves chosen for themselves, what is there wonderful if He did also at that time give over to their unbelief, Pharaoh, who never would have believed, along with those who were with him? As the Word spoke to Moses from the bush: “And I am sure that the king of Egypt will not let you go, unless by a mighty hand.” And for the reason that the Lord spoke in parables, and brought blindness upon Israel, that seeing they might not see, since He knew the [spirit of] unbelief in them, for the same reason did He harden Pharaoh's heart; in order that, while seeing that it was the finger of God which led forth the people, he might not believe, but be precipitated into a sea of unbelief, resting in the notion that the exit of these [Israelites] was accomplished by magical power, and that it was not by the operation of God that the Red Sea afforded a passage to the people, but that this occurred by merely natural causes (sed naturaliter sic se habere).
Source: Adversus haereses (New Advent)