Augustine of Hippo

(Serm. 82. 1.) Our Lord admonishes us not to overlook one another's faults, yet not so as seeking for matter of blame, but watching what you may amend. For our rebuke should be in love, not eager to wound, but anxious to amend. If you pass it by, you are become worse than he. He by doing you a wrong hath done himself a great hurt; you slight your brother's wound, and are more to blame for your silence than he for his ill words to you.

(de Civ. Dei, i. 9.) For often we wrongly shun to teach and admonish, or to rebuke and check the wicked, either because the task is irksome, or because we would escape their enmity, lest they should harm or obstruct us in temporal things, whether in gaining objects we desire, or in holding what our frailty fears to love. But if any one spares reproof of evil doers, because he seeks fitter occasion, or fears to make them worse, or that they may be an impediment to the good and pious living of other weak ones, or may grieve them, or turn them from the faith; herein there is seen no considerations of covetousness, but the prudence of charity. And much weightier reason have they who are set over the churches, to the end they should not spare to rebuke sin; though not even he is free from this blame, who, though not in authority, wots of many things in them to whom he is bound by the ties of this life, which should be touched by admonition or correction, but neglects to do so; shunning their displeasure on account of things which he does not unduly use in this life, but wherewith he is unduly delighted.

(Serm. 82, 7.) When any one therefore offends against us, let us be very careful, not for ourselves, for it is glorious to forget an injury; forget therefore your own wrong, but not the wound your brother has sustained; and tell him of his fault between him and you alone, seeking his amendment and sparing his shame. For it may be that out of shame he will seek to defend his fault, and thus you will only harden, while you sought to do him good.

(ubi sup.) But the Apostle says, Them that sin 'rebuke before all, that others may fear to do the like. (1 Tim 5:20.) Sometimes therefore your brother is to be spoken to between thee and him alone, sometimes to be rebuked before all. What you must do first, attend and learn; If thy brother, says He, sin against thee, tell him of his fault between thee and him alone. Why? Because he has sinned against you? What is it that he has sinned against you? You know that he has sinned, and therefore since his sin was in private, let your rebuke be in private too. For if you alone know of his trespass, and proceed to rebuke him before all, you do not correct but betray him. Your brother has sinned against you; if you alone know thereof, then he has sinned against you only; but if he did you a wrong in the presence of many, then he has sinned against those also who were witnesses of his fault. Those faults then are to be rebuked before all, that are committed before all; those which are done in private, are to be rebuked in private. Discern times, and the Scriptures are consistent. But why do you correct your neighbour? Because his trespass has hurt yourself? Far be it from thee. If you do it from self-love, you do nought; if you do it from love of him, you do most rightly. Lastly, in what you shall say to him, keep in view for whose sake it is that you ought to do it, for your own or for his, for it follows, If he hear thee, thou hast gained thy brother; do it therefore for his sake, that you may gain him. And do you confess that by your sin against man you were lost; for if you were not lost, how has he gained you? Let none then make light of it when he sins against his brother.

If someone has done you injury and you have suffered, what should be done? You have heard the answer already in today’s Scripture: “If your brother sins against you, go and tell him his fault, between you and him alone.”If you fail to do so, you are worse than he is. He has done someone harm, and by doing harm he has stricken himself with a grievous wound. Will you then completely disregard your brother’s wound? Will you simply watch him stumble and fall down? Will you disregard his predicament? If so, you are worse in your silence than he in his abuse.
Therefore, when any one sins against us, let us take great care, but not merely for ourselves. For it is a glorious thing to forget injuries. Just set aside your own injury, but do not neglect your brother’s wound. Therefore “go and tell him his fault, between you and him alone,” intent upon his amendment but sparing his sense of shame. For it might happen that through defensiveness he will begin to justify his sin, and so you will have inadvertently nudged him still closer toward the very behavior you desire to amend. Therefore “tell him his fault between you and him alone. If he listens to you, you have gained your brother,” because he might have been lost, had you not spoken with him.

What then does "seventy times seven" mean? Listen, my friends, to this great mystery, this wonderful gift. When the Lord was baptized, Luke the holy Evangelist there noted down his ancestry, in what order, series and stems that generation had been reached in which Christ was born. Matthew began from Abraham and came as far as Joseph in descending generations. But Luke began to count by going back up in ascending order. Why does the one reckon in a descending and the other in an ascending order? Because Matthew set forth the generations of Christ by which he came down to us, and so he began to reckon when Christ was born from a descending order. Luke begins to reckon in reverse from when Christ was baptized. In this is the beginning of an ascension, for he begins to reckon in an ascending order. Note that in his account he enumerates seventy-seven generations! With whom did he begin his reckoning? Note carefully! He began to reckon from Christ up to Adam himself, who was the first sinner and who parented us into bondage to sin. Luke reckoned up to Adam, and so there are enumerated in toto seventy-seven generations—from Christ up to Adam and from Adam up to Christ. Note seventy-seven! So then if no generation was omitted, there is no exemption of any trespass that ought not to be forgiven. For therefore did he reckon up his seventy-seven generations, which number the Lord mentioned as to the forgiveness of sins; since he begins to reckon from the baptism, where all sins are remitted.