They who have committed a “sin unto death” are not to be abandoned, but subjected to penance, according to St. Paul. Explanation of the phrase “Deliver unto Satan.” Satan can afflict the body, but these afflictions bring spiritual profit, showing the power of God, Who thus turns Satan's devices against himself.
59. Lastly, Paul teaches us that we must not abandon those who have committed a sin unto death, but that we must rather coerce them with the bread of tears and tears to drink, yet so that their sorrow itself be moderated. For this is the meaning of the passage: “You have given them to drink in large measure,” that their sorrow itself should have its measure, lest perchance he who is doing penance should be consumed by overmuch sorrow, as was said to the Corinthians: “What will you? Shall I come to you with a rod, or in love and a spirit of meekness?” But even the rod is not severe, since he had read: “You shall beat him indeed with the rod, but shall deliver his soul from death.”
60. What the Apostle means by the rod is shown by his invective against fornication, his denunciation of incest, his reprehension of pride, because they were puffed up who ought rather to be mourning, and lastly, his sentence on the guilty person, that he should be excluded from communion, and delivered to the adversary, not for the destruction of the soul but of the flesh. For as the Lord did not give power to Satan over the soul of holy Job, but allowed him to afflict his body, so here, too, the sinner is delivered to Satan for the destruction of the flesh, that the serpent might lick the dust of his flesh, but not hurt his soul.
61. Let, then, our flesh die to lusts, let it be captive, let it be subdued, and not war against the law of our mind, but die in subjection to a good service, as in Paul, who buffeted his body that he might bring it into subjection, in order that his preaching might become more approved, if the law of his flesh agreed and was consonant with the law of his flesh. For the flesh dies when its wisdom passes over into the spirit, so that it no longer has a taste for the things of the flesh, but for the things of the spirit. Would that I might see my flesh growing weak, would that I were not dragged captive into the law of sin, would that I lived not in the flesh, but in the faith of Christ! And so there is greater grace in the infirmity of the body than in its soundness.
62. Having explained Paul's meaning, let us now consider the words themselves, in what sense he said that he had delivered him to Satan for the destruction of the flesh, for the devil it is who tries us. For he brings ailments on each of our limbs, and sickness on our whole bodies. And then, too, he smote holy Job with evil sores from the feet to the head, because he had received the power of destroying his flesh, when God said: “Behold, I give him up unto you, only preserve his life.” This the Apostle took up in the same words, giving up this man to Satan for the destruction of the flesh, that his spirit might be saved in the day of our Lord Jesus Christ.
63. Great is the power, great is the gift, which commands the devil to destroy himself. For he destroys himself when he makes the man whom he is seeking to overthrow by temptation stronger instead of weak, because while he is weakening the body he is strengthening his soul. For sickness of the body restrains sin, but luxury sets on fire the sin of the flesh.
64. The devil is then deceived so as to wound himself with his own bite, and to arm against himself him whom he thought to weaken. So he armed holy Job the more after he wounded him, who, with his whole body covered with sores, endured indeed the bite of the devil, but felt not his poison. And so it is well said of him, “You shall draw out the dragon with an hook, you will play with him as with a bird, you shall bind him as a boy does a sparrow, you shall lay your hand upon him.”
65. You see how he is mocked by Paul, so that, like the child in prophecy, he lays his hand on the hole of the asp, and the serpent injures him not; he draws him out of his hiding-places, and makes of his venom a spiritual antidote, so that what is venom becomes a medicine, the venom serves to the destruction of the flesh, it becomes medicine to the healing of the spirit. For that which hurts the body benefits the spirit.
66. Let, then, the serpent bite the earthy part of me, let him drive his tooth into my flesh, and bruise my body; and may the Lord say of me: “I give him up unto you, only preserve his life.” How great is the power of Christ, that the guardianship of man is made a charge even to the devil himself, who always desires to injure him. Let us then make the Lord Jesus favourable to ourselves. At the command of Christ the devil himself becomes the guardian of his prey. Even unwillingly he carries out the commands of heaven, and, though cruel, obeys the commands of gentleness.
67. But why do I commend his obedience? Let him be ever evil that God may be ever good, Who converts his ill-will into grace for us. He wishes to injure us, but cannot if Christ resist him. He wounds the flesh but preserves the life. And then it is written: “Then shall the wolves and the lambs feed together, the lion and the ox shall eat straw, and they shall not hurt nor destroy in My holy mountain, says the Lord.” For this is the sentence of condemnation on the serpent: “Dust shall be your food.” What dust? Surely that of which it is said: “Dust you are, and into dust shall you return.”
Source: On Repentance (New Advent)