4 Therefore ought we to rebuke in love; not with any eager desire to injure, but with an earnest care to amend. If we be so minded, most excellently do we practise that which we have been recommended today; “If your brother shall sin against you, rebuke him between you and him alone.” Why do you rebuke him? Because you are grieved, that he should have sinned against you? God forbid. If from love of yourself you do it, you do nothing. If from love to him you do it, you do excellently.
In fact, observe in these words themselves, for the love of whom you ought to do it, whether of yourself or him. “If he shall hear you, you have gained your brother.” Do it for his sake then, that you may “gain” him. If by so doing you “gain” him, had you not done it, he would have been lost. How is it then that most men disregard these sins, and say, “What great thing have I done? I have only sinned against man.” Disregard them not. You have sinned against man; but would you know that in sinning against man you are lost.
If he, against whom you have sinned, have “rebuked you between you and him alone,” and you have listened to him, he has “gained” you. What can “has gained you,” mean; but that you had been lost, if he had not gained you. For if you would not have been lost, how has he gained you? Let no man then disregard it, when he sins against a brother. For the Apostle says in a certain place, “But when you sin so against the brethren, and wound their weak conscience, you sin against Christ;” for this reason, because we have been all made members of Christ. How do you not sin against Christ, who sinnest against a member of Christ?
Source: Sermons on Selected Lessons of the New Testament (New Advent)