Large Catechism: Public Sin (LC I 267-278)

But you say, “Shall I not say something if it is the truth?”Answer: “Why do you not make your accusation to regular judges?”“Ah, I cannot prove it publicly, and so I might be silenced and turned away in a harsh manner.”“Ah, indeed, do you smell the roast?”If you do not trust yourself to stand before the proper authorities and to answer well, then hold your tongue. But if you know about it, know it for yourself and not for another. For if you tell the matter to others—although it is true—you will look like a liar, because you cannot prove it. Besides, you are acting like a rascal. We should never deprive anyone of his honor or good name unless it is first taken away from him publicly. (270)