10 You will soon see, Beloved, what we ought to do, and when; only I would we may not be slow to practise it. Attend and see: “If your brother sin against you, rebuke him between him and you alone.” Why? Because it is against you that he has sinned. What is that, “has sinned against you”? You know that he has sinned. For because it was secret when he sinned against you, seek for secresy, when you correct his sin. For if you only know that he has sinned against you, and you would “rebuke him before all,” you are not a reprover, but a betrayer.
Consider how that “just man” Joseph spared his wife with such exceeding kindness, in so great a crime as he had suspected her of, before he knew by whom she had conceived; because he perceived that she was with child, and he knew that he had not come in unto her. There remained then an unavoidable suspicion of adultery, and yet because he only had perceived, he only knew it, what does the Gospel say of him? “Then Joseph being a just man, and not willing to make her a public example.” The husband's grief sought no revenge; he wished to profit, not to punish the sinner.
“And not willing to make her a public example, he was minded to put her away privily.” But while he thought on these things, “behold, the Angel of the Lord appeared unto him,” in sleep; and told him how it was, that she had not defiled her husband's bed, but that she had conceived of the Holy Ghost the Lord of them both. Your brother then has sinned against you; if you alone know it, then has he really sinned against you alone. For if in the hearing of many he has done you an injury, he has sinned against them also whom he has made witnesses of his iniquity.
For I tell you, my dearly beloved Brethren, what you can yourselves recognise in your own case. When any one does my brother an injury in my hearing, God forbid that I should think that injury unconnected with myself. Certainly he has done it to me also; yea to me the rather, to whom he thought what he did was pleasing. Therefore those sins are to be reproved before all, which are committed before all; they are to be reproved with more secresy, which are committed more secretly. Distinguish times, and Scripture is in harmony with itself.
Source: Sermons on Selected Lessons of the New Testament (New Advent)