Large Catechism: Am I Called to Speak Out? (LC I 279-291)

All this has been said about secret sins. But where the sin is quite public, so that the judge and everybody know about it, you can without any sin shun the offender and let him go his own way, because he has brought himself into disgrace. You may also publicly testify about him. For when a matter is public in the daylight, there can be no slandering or false judging or testifying. It is like when we now rebuke the pope with his doctrine, which is publicly set forth in books and proclaimed in all the world. Where the sin is public, the rebuke also must be public, that everyone may learn to guard against it. (284)