1. Men sin by deviating from God their last end. But as every agent acts to its own proper and befitting end, it is impossible for God's action to avert any from their ultimate end in God.
2. Good cannot be the cause of evil, nor God the cause of sin.
3. All the wisdom and goodness of man is derived from the wisdom and goodness of God, being a likeness thereof. But it is repugnant to the wisdom and goodness of man to make any one to sin: therefore much more to divine wisdom and goodness.
4. A fault always arises from some defect of the proximate agent, not from any defect of the prime agent. Thus the fault of limping comes from some defect of the shin-bone, not from the locomotor power, from which power however is whatever perfection of movement appears in the limping. But the proximate agent of human sin is the will. The sinful defect then is from the will of man, not from God, who is the prime agent, of whom however is whatever point of perfect action appears in the act of sin.*
Hence it is said: Say not, He himself hath led me astray: for he hath no use for sinful men: He hath commanded none to do impiously, and he hath not given to any man license to sin (Ecclus xv, 12, 21): Let none, when he is tempted, say that he is tempted by God: for God tempteth no man to evil (James i, 13).
Still there are passages of Scripture, from which it might seem that God is to some men the cause of sin. Thus it is said: I have hardened the heart of Pharaoh and his servants (Exod. x, 1): Blind the heart of this people, and make its ears dull, and close its eyes, lest perchance it see with its eyes, and be converted, and I heal it: Thou hast made us wander from thy ways: Thou hast hardened our heart, that we should not fear thee (Isai. vi, 10: lxiii, 17): God delivered them over to a reprobate sense, to do those things which are not seemly (Rom. i, 28). All these passages are to be understood as meaning that God does not bestow on some the help for avoiding sin which He bestows on others. This help is not merely the infusion of grace, but also an exterior guardianship, whereby the occasions of sin are providentially removed from a man's path. God also aids man against sin by the natural light of reason, and other natural goods that He bestows on man.* When then He withdraws these aids from some, as their conduct deserves that he should, according to the exigency of His justice, He is said to harden them, or to blind them.
3.162 : That some Men God delivers from Sin, and some He leaves in Sin
3.164 : Of Predestination, Reprobation, and Divine Election