We have one more treasure which is indispensable to us, namely our honour and good name; for it is intolerable to live among people in public disgrace and contempt. (Proverbs 22:1) Therefore, God will not have our neighbour deprived of his reputation, honour and character any more than of his money and possessions; (Mark 14:3-9) He would have every man maintain his self-respect before his wife, children, servants, and neighbours. In its first and simplest meaning…this commandment pertains to public courts of justice where a poor, innocent man is accused and maligned by false witnesses (Colossians 3:9) [Concordia 399:255-257]
It forbids all sins of the tongue by which we may injure or offend our neighbour…It applies particularly to the detestable, shameful vice of back-biting or slander by which the devil rides us…It is a common vice of human nature that everyone would rather hear evil than good about his neighbour (Proverbs 11:13) [400:262-264].
Nobody has the right to judge and reprove his neighbour publicly, even when he has seen a sin committed, unless he has been authorised to judge and reprove. There is a great difference between judging sin and having knowledge of sin. Knowledge of sin does not entail the right to judge it. I may see and hear that my neighbour sins, but to make him the talk of the town is not my business. If I interfere and pass sentence on him, I fall into a greater sin than his. When you become aware of a sin, simply make your ears a tomb and bury it until you are appointed a judge and authorised to administer punishment by virtue of your office [400-401: 264-266]
All this refers to secret sins. But where the sin is so public that the judge and the whole world are aware of it…you may testify publicly concerning him.