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).

Christ worried about this, and not only worried, but also predicted that it would happen. The world refuses to be changed, even if we preached ourselves to death. Wherever the Gospel arises, therefore, the factions and sects must follow, to spoil it and put it down. The reason is that the devil must sow his seed among the good seed (Matt. 13:25); and wherever God builds a church, he builds his chapel or tabernacle next to it.² Satan always wants to be in the midst of the children of God, as Scripture says (Job 1:6). Christ intends this as a warning to His apostles and loyal preachers. They must maintain a diligent watch against this vice and be careful not to let it intrude itself and cause schisms and dissension, especially in doctrine. It is as if He were to say: “If you want to be My disciples, let your understanding and your ideas in doctrine all be the same. Let no one claim to be the master who knows something new or better, judging or condemning the others. Concentrate on what I command you to preach, not on who is to do the preaching. And preserve harmony among yourselves, so that one does not despise the other or start something new.”  (Luther’s Works, v. 21, pages 212).