Reprobation: The Awesome Power of God: Calvin's Calvinism - Section VI

So, when Pharaoh is said to have been "hardened" of God, he was already, in himself, worthy of being delivered over unto Satan by the Most High. Moses, however, also testifies that Pharaoh had been before blinded of God "for this very purpose" (Exodus 9:16). Nor does Paul add any other cause for this, than that Pharaoh was one of the reprobate (Romans 9:17).  -- John Calvin

Although, therefore, the Lord doth thus strike the wicked with vindictive madness and consternation, and doth thus repay them with the punishment they deserve; yet this does not at all alter the fact that there is, in all the reprobate generally, a blindness and an obstinate hardness of heart. So, when Pharaoh is said to have been "hardened" of God, he was already, in himself, worthy of being delivered over unto Satan by the Most High. Moses, however, also testifies that Pharaoh had been before blinded of God "for this very purpose" (Exodus 9:16). Nor does Paul add any other cause for this, than that Pharaoh was one of the reprobate (Romans 9:17). In this same manner also does the apostle demonstrate that the Jews, when God had deprived them of the light of understanding, and had permitted them to fall into horrible darkness, suffered thereby the righteous punishments of their wicked contempt of the grace of God. And yet the apostle plainly intimates that this same blindness is justly inflicted of God upon all reprobates generally. For he testifies that the "remnant were saved "according to the election of grace," but that all "the rest were blinded." If, then, all "the rest," in the salvation of whom the election of God does not reign, are "blinded," it is doubtlessly and undeniably manifest that those same persons who, by their rebellion and provocation of the wrath of God, procured to themselves this additional blindness, were themselves from the beginning ordained to blindness. Hence the words of Paul are manifestly true, where he says that the vessels of wrath were "afore prepared unto destruction"; namely, all those who, being destitute of the Spirit of adoption, precipitated themselves into eternal destruction by their own sin and fault. Wherefore, I hesitate not to confess that in the secret judgments of God something always precedes, but "hidden." For how God condemns the wicked, and yet justifies the wicked, is a mystery that is shut up in that secret mind of God, which is inaccessible to all human understanding. Wherefore, there remains nothing better, nothing more becoming us, than to stand in awe with the apostle, and exclaim, "How unsearchable are His judgments, and His ways past finding out!" (Romans 11:33) For God's judgments are a profound abyss.

Calvin's Calvinism - Section VI