You must understand this in such a way that it does not take away the right of the man in the public ministry of preaching to judge matters of doctrine as well as of life. Indeed, it is incumbent on him in his office to rebuke publicly whatever does not square with true doctrine, for the very purpose of preventing sects from coming in and taking hold. When he sees that life is wrong, he must likewise denounce it and resist it. He is put there to oversee this, and he will have to give account of it (Heb. 13:17). In fact, whenever any Christian sees his neighbor doing wrong, he has the duty of admonishing him and restraining him, which is impossible without judging and passing judgment. But this is all done on the basis of an office or a commission, which Christ, is not discussing here, as we have said often enough. What is forbidden is that everyone may go ahead on the basis of his own ideas and make a doctrine and spirit of his own, imagining that he is to be Master Smart Aleck, who is supposed to correct everybody and to criticize him, though he has no commission to do anything of the sort. These are the people that the Lord is denouncing here. He does not want anyone to undertake or to do anything on the basis of his own ideas and without a commission, especially not the task of judging other people.
(Luther’s Works, v. 21, pages 212-213).