He enables the one on whom he has mercy to do good, and he leaves the one whom he hardens to do evil. But that mercy is the result of the prior merit of faith, and that hardening is the fruit of prior unbelief, so that we do good deeds by the gift of God and evil deeds because of his punishment. Yet in either case free will is not taken away from man, whether it is to believe in God, so that mercy on us might follow, or to disbelieve in him, so that punishment on us might be the result.
You must believe that the man whom God permits to go astray and to become hardened has deserved this evil, while in the case of the man upon whom he has mercy, you must acknowledge with an unswerving faith that this is a case of the grace of God, who is rendering not evil for evil but good for evil.
Why does the Father not teach all people in order that they might come to Christ, unless it is that all those whom he teaches, he teaches because of mercy, but those whom he does not teach, he does not teach because of judgment? Predestination of the Saints
And whom he will, he hardeneth. That is, permits to be hardened by their own malice, as it is divers times said in Exodus, that Pharao hardened his heart. God, says St. Augustine, is said to harden men's hearts, not by causing their malice, but by not giving them the free gift of his grace, by which they become hardened by their own perverse will. (Witham)
Not by being the cause, or author of his sin, but by withholding his grace, and so leaving him in his sin, in punishment of his past demerits. (Challoner)
