Objection 1. It would seem that blindness and hardness of heart are always directed to the salvation of those who are blinded and hardened. For Augustine says (Enchiridion xi) that "as God is supremely good, He would nowise allow evil to be done, unless He could draw some good from every evil." Much more, therefore, does He direct to some good, the evil of which He Himself is the cause. Now God is the cause of blindness and hardness of heart, as stated above (Article 3). Therefore they are directed to the salvation of those who are blinded and hardened.
Objection 2. Further, it is written (Wisdom 1:13) that "God hath no pleasure in the destruction of the ungodly [Vulgate: 'God made not death, neither hath He pleasure in the destruction of the living.']." Now He would seem to take pleasure in their destruction, if He did not turn their blindness to their profit: just as a physician would seem to take pleasure in torturing the invalid, if he did not intend to heal the invalid when he prescribes a bitter medicine for him. Therefore God turns blindness to the profit of those who are blinded.
Objection 3. Further, "God is not a respecter of persons" (Acts 10:34). Now He directs the blinding of some, to their salvation, as in the case of some of the Jews, who were blinded so as not to believe in Christ, and, through not believing, to slay Him, and afterwards were seized with compunction, and converted, as related by Augustine (De Quaest. Evang. iii). Therefore God turns all blindness to the spiritual welfare of those who are blinded.
Objection 4. On the other hand, according to Romans 3:8, evil should not be done, that good may ensue. Now blindness is an evil. Therefore God does not blind some for the sake of their welfare.
I answer that, Blindness is a kind of preamble to sin. Now sin has a twofold relation--to one thing directly, viz. to the sinner's damnation--to another, by reason of God's mercy or providence, viz. that the sinner may be healed, in so far as God permits some to fall into sin, that by acknowledging their sin, they may be humbled and converted, as Augustine states (De Nat. et Grat. xxii). Therefore blindness, of its very nature, is directed to the damnation of those who are blinded; for which reason it is accounted an effect of reprobation. But, through God's mercy, temporary blindness is directed medicinally to the spiritual welfare of those who are blinded. This mercy, however, is not vouchsafed to all those who are blinded, but only to the predestinated, to whom "all things work together unto good" (Romans 8:28). Therefore as regards some, blindness is directed to their healing; but as regards others, to their damnation; as Augustine says (De Quaest. Evang. iii).
Reply to Objection 1. Every evil that God does, or permits to be done, is directed to some good; yet not always to the good of those in whom the evil is, but sometimes to the good of others, or of the whole universe: thus He directs the sin of tyrants to the good of the martyrs, and the punishment of the lost to the glory of His justice.
Reply to Objection 2. God does not take pleasure in the loss of man, as regards the loss itself, but by reason of His justice, or of the good that ensues from the loss.
Reply to Objection 3. That God directs the blindness of some to their spiritual welfare, is due to His mercy; but that the blindness of others is directed to their loss is due to His justice: and that He vouchsafes His mercy to some, and not to all, does not make God a respecter of persons, as explained in the I, 23, 5, ad 3.
Reply to Objection 4. Evil of fault must not be done, that good may ensue; but evil of punishment must be inflicted for the sake of good.
Source: Summa Theologica (New Advent)