Objection 1. It would seem that God is not the cause of spiritual blindness and hardness of heart. For Augustine says (Qq. lxxxiii, qu. 3) that God is not the cause of that which makes man worse. Now man is made worse by spiritual blindness and hardness of heart. Therefore God is not the cause of spiritual blindness and hardness of heart.
Objection 2. Further, Fulgentius says (De Dupl. Praedest. i, 19): "God does not punish what He causes." Now God punishes the hardened heart, according to Sirach 3:27: "A hard heart shall fear evil at the last." Therefore God is not the cause of hardness of heart.
Objection 3. Further, the same effect is not put down to contrary causes. But the cause of spiritual blindness is said to be the malice of man, according to Wisdom 2:21: "For their own malice blinded them," and again, according to 2 Corinthians 4:4: "The god of this world hath blinded the minds of unbelievers": which causes seem to be opposed to God. Therefore God is not the cause of spiritual blindness and hardness of heart.
On the contrary, It is written (Isaiah 6:10): "Blind the heart of this people, and make their ears heavy," and Romans 9:18: "He hath mercy on whom He will, and whom He will He hardeneth."
I answer that, Spiritual blindness and hardness of heart imply two things. One is the movement of the human mind in cleaving to evil, and turning away from the Divine light; and as regards this, God is not the cause of spiritual blindness and hardness of heart, just as He is not the cause of sin. The other thing is the withdrawal of grace, the result of which is that the mind is not enlightened by God to see aright, and man's heart is not softened to live aright; and as regards this God is the cause of spiritual blindness and hardness of heart.
Now we must consider that God is the universal cause of the enlightening of souls, according to John 1:9: "That was the true light which enlighteneth every man that cometh into this world," even as the sun is the universal cause of the enlightening of bodies, though not in the same way; for the sun enlightens by necessity of nature, whereas God works freely, through the order of His wisdom. Now although the sun, so far as it is concerned, enlightens all bodies, yet if it be encountered by an obstacle in a body, it leaves it in darkness, as happens to a house whose window-shutters are closed, although the sun is in no way the cause of the house being darkened, since it does not act of its own accord in failing to light up the interior of the house; and the cause of this is the person who closed the shutters. On the other hand, God, of His own accord, withholds His grace from those in whom He finds an obstacle: so that the cause of grace being withheld is not only the man who raises an obstacle to grace; but God, Who, of His own accord, withholds His grace. In this way, God is the cause of spiritual blindness, deafness of ear, and hardness of heart.
These differ from one another in respect of the effects of grace, which both perfects the intellect by the gift of wisdom, and softens the affections by the fire of charity. And since two of the senses excel in rendering service to the intellect, viz. sight and hearing, of which the former assists "discovery," and the latter, "teaching," hence it is that spiritual "blindness" corresponds to sight, "heaviness of the ears" to hearing, and "hardness of heart" to the affections.
Reply to Objection 1. Blindness and hardheartedness, as regards the withholding of grace, are punishments, and therefore, in this respect, they make man no worse. It is because he is already worsened by sin that he incurs them, even as other punishments.
Reply to Objection 2. This argument considers hardheartedness in so far as it is a sin.
Reply to Objection 3. Malice is the demeritorious cause of blindness, just as sin is the cause of punishment: and in this way too, the devil is said to blind, in so far as he induces man to sin.
Source: Summa Theologica (New Advent)