[Jesus] addresses here particularly two organs of the body, the ear and the tongue; for you know the Kingdom of Christ is founded upon the Word, which cannot be apprehended or understood except by these two organs, the ear and the tongue, and he rules in the hearts of men alone by the Word and by faith. The ears apprehend the Word, the heart believes it; the tongue, however, speaks or confesses that which the heart believes. Hence, barring the tongue and ears, there is no perceptible difference between the Kingdom of Christ and that of the world.
For in regard to the outward life a Christian has duties like an unbeliever; he tills the ground, works his fields, and plows just like others, and he undertakes no peculiar work or deed, either in eating, drinking, working, sleeping, or anything else. But these two organs of the body make a difference between a Christian and an unbeliever; a Christian speaks and hears differently; he has a tongue which praises the grace of God and preaches Christ the Lord as being the only Savior, etc. This the world does not do; it speaks of avarice and other vices, preaches and praises its own glory.
In like manner the ears of both differ. A Christian’s ears have the same Word which the tongue preaches, and the heart believes; but the world prefers to hear one speak of her wisdom, understanding, honor and glory. The ears and tongues of Christians are thus different from the ears and tongues of the world, or of unbelievers, caring naught for silver or gold, but only for that which is said of Christ, and how to speak and preach Christ.
– From Luther’s House Postils