12 What, then, will you do? Tell me. To fulfill the law in every part, so as to offend in nothing, is difficult: the condition of guilt is therefore certain; will you refuse to use the remedy? Behold, my brethren, what a remedy the Lord has provided for the sicknesses of the soul! What then? When your head aches, we praise you if you place the gospel at your head, instead of having recourse to an amulet. For so far has human weakness proceeded, and so lamentable is the estate of those who have recourse to amulets, that we rejoice when we see a man who is upon his bed, and tossed about with fevers and pains, placing his hope on nothing else than that the gospel lies at his head; not because it is done for this purpose, but because the gospel is preferred to amulets.
If, then, it is placed at the head to allay the pain of the head, is it not placed at the heart to heal it from sin? Let it be done then. Let what be done? Let it be placed at the heart, let the heart be healed. It is well—well that you should have no further care regarding the safety of the body, than to ask it from God. If He knows that it will do you good, He will give it you; if He give it not to you, it would not have profited you to have it. How many are sick in bed, and for that reason are innocent!
For if they were to recover, they would go forth to commit acts of wickedness. To how many is health an injury! The robber who goes forth to the narrow path to slay a man, how much better for him would it have been to have been sick! And he who rises by night to dig through his neighbor's wall, how much better for him to be tossed by fever! If he were ill, he would have been comparatively innocent; being well, he is guilty of wickedness. It is known, then, to God what is expedient for us: let us make this only our endeavor, that our hearts be whole from sins; and when it happens that we are scourged in the body, let us pray to Him for relief.
The Apostle Paul besought Him that He would take away the thorn in his flesh, and He would not. Was he disturbed? Was he filled with sadness, and did he speak of himself as deserted? Rather did he say that he was not deserted, because that was not taken away which he desired to be taken away, to the end that infirmity might be cured. For this he found in the voice of the Physician, “My grace is sufficient for you; for my strength is made perfect in weakness.” Whence do you know, then, that God does not wish to heal you?
As yet it is expedient for you to be scourged. Whence do you know how diseased that is which the physician cuts, using his knife on the diseased parts? Does he not know the measure, what he is to do, and how far he is to do it? Does the shrieking of him he cuts restrain the hands of the physician cutting according to his art? The one cries, the other cuts. Is he cruel who does not listen to the man crying out, or is he not rather merciful in following the wound, that he may heal the sick man? These things have I said, my brethren, in order that no one seek any other aid than that of God, when we happen to be under the reproof of God. See that you perish not; see that you do not depart from the Lamb, and be devoured by the lion.
Source: Tractates on the Gospel of John (New Advent)