The Lord employs such vivid action [in Mark chapter seven] for the sake of the spiritual miracle. He wants to demonstrate how great an effort is required to cause a deaf man to hear and a mute man to speak. Lazarus he waked up with a word. To the palsied man he said, “Stand up and walk!” and he was healed. But with the deaf and dumb man he doesn’t proceed in such a clipped and simple way, but takes unusual steps by placing his fingers in the ears, sighing, and then saying, “Be opened!” He thereby shows us that if we are to be loosed from the devil’s bonds, and possess ready tongues and good ears, this can happen only through the external Word and preaching, through external means. We must, first of all, hear the Word, not neglecting baptism or the Sacrament either, and the Holy Spirit will then be present to free the ears and tongues.
Therefore, we must be on guard against the fantastic spirits who despise the external Word and Sacrament, waiting till God speaks to them in the heart. No, says Christ, here is my finger, the external Word, which must sound in the ears; my spittle, which must moisten and bestir the tongue; in this way my work proceeds rightly and readily from place to place. We see this wherever the external Word has free course; there true Christians will be found. Wherever it does not have free course, there none will be found, for as goes the shepherd, so the sheep.
Everyone should take care, therefore, to be found on this path and gladly hear God’s Word. Without the Word, God does not reveal himself in your heart. To see and know him can happen only through the external Word and Sacraments. The Holy Spirit works in no other way. This is what God taught at the time he spoke from heaven, “This is my beloved Son, hear ye him.” Likewise, Christ commands his disciples, “Go into all the world, teaching and baptizing all nations.” Again, “Whoever hears you, hears me.” Thus our dear Lord Jesus Christ commanded that we should open our mouths, preaching the gospel to people and baptizing them. This is the only way to salvation; otherwise all is lost and for nothing. “Whoever hears you,” he says, “hears me.”
– House Postil for Trinity 12 (Mark 7:31-37)