Objection 1. It would seem that fraternal correction is not an act of charity. For a gloss on Matthew 18:15, "If thy brother shall offend against thee," says that "a man should reprove his brother out of zeal for justice." But justice is a distinct virtue from charity. Therefore fraternal correction is an act, not of charity, but of justice.
Objection 2. Further, fraternal correction is given by secret admonition. Now admonition is a kind of counsel, which is an act of prudence, for a prudent man is one who is of good counsel (Ethic. vi, 5). Therefore fraternal correction is an act, not of charity, but of prudence.
Objection 3. Further, contrary acts do not belong to the same virtue. Now it is an act of charity to bear with a sinner, according to Galatians 6:2: "Bear ye one another's burdens, and so you shall fulfil the law of Christ," which is the law of charity. Therefore it seems that the correction of a sinning brother, which is contrary to bearing with him, is not an act of charity.
On the contrary, To correct the wrongdoer is a spiritual almsdeed. But almsdeeds are works of charity, as stated above (Question 32, Article 1). Therefore fraternal correction is an act of charity.
I answer that, The correction of the wrongdoer is a remedy which should be employed against a man's sin. Now a man's sin may be considered in two ways, first as being harmful to the sinner, secondly as conducing to the harm of others, by hurting or scandalizing them, or by being detrimental to the common good, the justice of which is disturbed by that man's sin.
Consequently the correction of a wrongdoer is twofold, one which applies a remedy to the sin considered as an evil of the sinner himself. This is fraternal correction properly so called, which is directed to the amendment of the sinner. Now to do away with anyone's evil is the same as to procure his good: and to procure a person's good is an act of charity, whereby we wish and do our friend well. Consequently fraternal correction also is an act of charity, because thereby we drive out our brother's evil, viz. sin, the removal of which pertains to charity rather than the removal of an external loss, or of a bodily injury, in so much as the contrary good of virtue is more akin to charity than the good of the body or of external things. Therefore fraternal correction is an act of charity rather than the healing of a bodily infirmity, or the relieving of an external bodily need. There is another correction which applies a remedy to the sin of the wrongdoer, considered as hurtful to others, and especially to the common good. This correction is an act of justice, whose concern it is to safeguard the rectitude of justice between one man and another.
Reply to Objection 1. This gloss speaks of the second correction which is an act of justice. Or if it speaks of the first correction, then it takes justice as denoting a general virtue, as we shall state further on (58, 5), in which sense again all "sin is iniquity" (1 John 3:4), through being contrary to justice.
Reply to Objection 2. According to the Philosopher (Ethic. vi, 12), prudence regulates whatever is directed to the end, about which things counsel and choice are concerned. Nevertheless when, guided by prudence, we perform some action aright which is directed to the end of some virtue, such as temperance or fortitude, that action belongs chiefly to the virtue to whose end it is directed. Since, then, the admonition which is given in fraternal correction is directed to the removal of a brother's sin, which removal pertains to charity, it is evident that this admonition is chiefly an act of charity, which virtue commands it, so to speak, but secondarily an act of prudence, which executes and directs the action.
Reply to Objection 3. Fraternal correction is not opposed to forbearance with the weak, on the contrary it results from it. For a man bears with a sinner, in so far as he is not disturbed against him, and retains his goodwill towards him: the result being that he strives to make him do better.
Source: Summa Theologica (New Advent)