Matthew 7:6. Do not give dogs what is holy; and do not throw your pearls before swine, lest they trample them underfoot and turn to attack you.
Now the Lord Christ has almost finished His preaching about the fruit and the works that follow doctrine. Next He will begin warning and admonishing us to be on our guard against other doctrine. Thus He also admonishes the apostles when He sends them out to preach, and says (Matt. 10.16): “Behold, I send you out as sheep in the midst of wolves; so be wise as serpents and innocent as doves.” The situation of a Christian who is supposed to speak and preach the Word of God and to confess it by his life is really a dangerous one, because of the people. He has good reason to become impatient, because the world is so infinitely evil and because he lives in it surrounded by snakes and all kinds of vermin. That is why He declares: “Be careful not to throw what is holy before swine and dogs, for they might trample it underfoot or turn to attack you.” By this He intends to show them and to teach them a lesson. Wherever they go to preach in public before a crowd, they will find dogs and swine, who cannot do anything but trample on the Gospel and then persecute the preachers. (Luther’s Works, v. 21, p. 223-224).