3 But in the case of human infirmities, it is not easy in the first place for a man to discern them, for no man “knows the things of a man, save the spirit of man which is in him.” How then can any one apply the remedy for the disease of which he does not know the character, often indeed being unable to understand it even should he happen to sicken with it himself? And even when it becomes manifest, it causes him yet more trouble: for it is not possible to doctor all men with the same authority with which the shepherd treats his sheep.
For in this case also it is necessary to bind and to restrain from food, and to use cautery or the knife: but the reception of the treatment depends on the will of the patient, not of him who applies the remedy. For this also was perceived by that wonderful man (St. Paul) when he said to the Corinthians— “Not for that we have dominion over your faith, but are helpers of your joy.” For Christians above all men are not permitted forcibly to correct the failings of those who sin.
Secular judges indeed, when they have captured malefactors under the law, show their authority to be great, and prevent them even against their will from following their own devices: but in our case the wrong-doer must be made better, not by force, but by persuasion. For neither has authority of this kind for the restraint of sinners been given us by law, nor, if it had been given, should we have any field for the exercise of our power, inasmuch as God rewards those who abstain from evil by their own choice, not of necessity.
Consequently much skill is required that our patients may be induced to submit willingly to the treatment prescribed by the physicians, and not only this, but that they may be grateful also for the cure. For if any one when he is bound becomes restive (which it is in his power to be), he makes the mischief worse; and if he should pay no heed to the words which cut like steel, he inflicts another wound by means of this contempt, and the intention to heal only becomes the occasion of a worse disorder. For it is not possible for any one to cure a man by compulsion against his will.
Source: On the Priesthood (New Advent)