7 This then ought one to do who has done an injury. And he who has suffered one, what ought he to do? What we have heard today, “If your brother shall sin against you, rebuke him between you and him alone.” If you shall neglect this, you are worse than he. He has done an injury, and by doing an injury, has stricken himself with a grievous wound; will you disregard your brother's wound? Will you see him perishing, or already lost, and disregard his case? You are worse in keeping silence, than he in his reviling.
Therefore when any one sins against us, let us take great care, not for ourselves, for it is a glorious thing to forget injuries; only forget your own injury, not your brother's wound. Therefore “rebuke him between you and him alone,” intent upon his amendment, but sparing his shame. For it may be that through shame-facedness he will begin to defend his sin, and so you will make him whom you desire to amend, still worse. “Rebuke him” therefore “between him and you alone. If he shall hear you, you have gained your brother;” because he would have been lost, had you not done it.
But “if he will not hear you,” that is, if he will defend his sin as if it were a just action, “take with you one or two more, that in the mouth of two or three witnesses every word may be established; and if he will not hear them, refer it to the Church; but if he will not hear the Church, let him be unto you as an heathen man and a publican.” Reckon him no more among the number of your brethren. But yet neither is his salvation on that account to be neglected. For the very heathen, that is, the Gentiles and Pagans, we do not reckon among the number of brethren; but yet are we ever seeking their salvation.
This then have we heard the Lord so advising, and with such great carefulness enjoining, that He even added this immediately, “Verily I say unto you, Whatsoever you shall bind on earth, shall be bound in heaven; and whatsoever you shall loose on earth, shall be loosed in heaven.” You have begun to hold your brother for a publican; “you bind him on earth;” but see that you bind him justly. For unjust bonds justice does burst asunder. But when you have corrected, and been “reconciled to your brother,” you have “loosed him on earth.” And when “you shall have loosed him on earth, he shall be loosed in heaven also.” Thus you do a great thing, not for yourself, but for him; for a great injury had he done, not to you, but to himself.
Source: Sermons on Selected Lessons of the New Testament (New Advent)