The Bible presents an astonishingly simple method of preaching. In Nehemiah 8:8 we read that Ezra and his fellow preachers “read from the book, from the Law of God, clearly, and they gave the sense, so that the people understood the reading.”
There is no calling as majestic as the Christian ministry, and yet the central task of ministry is breathtakingly uncomplicated. We read the Bible aloud, we read it clearly, and then we explain what we have read, so that hearers understand the meaning. Of course, no one said it was easy. This is an arduous calling, but it is incredibly simple in design.
The most amazing thing about preaching is the fact that God chose to use human mouths for his message. It is astounding that God has willed that the earth shall hear his voice by means of the human voice.
Martin Luther put it this way:
“Thus when you hear a sermon by St. Paul or by me, you hear God the Father Himself. And yet you do not become my pupil but the Father’s, for it is not I who is speaking; it is the Father. Nor am I your schoolmaster; but we both, you and I, have one Schoolmaster and Teacher, the Father, who instructs us. We both, pastor and listener, are only pupils; there is only this difference, that God is speaking to you through me. That is the glorious power of the divine Word, through which God Himself deals with us and speaks to us, and in which we hear God Himself.” 
In this light, perhaps the most clarifying way to understand the preacher’s task is to consider its most quintessential act — the opening of the mouth.
Look with me to the Book of Acts, 10:30-43:
And Cornelius said, “Four days ago, about this hour, I was praying in my house at the ninth hour, and behold, a man stood before me in bright clothing and said, ‘Cornelius, your prayer has been heard and your alms have been remembered before God. Send therefore to Joppa and ask for Simon who is called Peter. He is lodging in the house of Simon, a tanner, by the sea.’ So I sent for you at once, and you have been kind enough to come. Now therefore we are all here in the presence of God to hear all that you have been commanded by the Lord.” So Peter opened his mouth and said: “Truly I understand that God shows no partiality, but in every nation anyone who fears him and does what is right is acceptable to him. As for the word that he sent to Israel, preaching good news of peace through Jesus Christ (he is Lord of all), you yourselves know what happened throughout all Judea, beginning from Galilee after the baptism that John proclaimed: how God anointed Jesus of Nazareth with the Holy Spirit and with power. He went about doing good and healing all who were oppressed by the devil, for God was with him. And we are witnesses of all that he did both in the country of the Jews and in Jerusalem. They put him to death by hanging him on a tree, but God raised him on the third day and made him to appear, not to all the people but to us who had been chosen by God as witnesses, who ate and drank with him after he rose from the dead. And he commanded us to preach to the people and to testify that he is the one appointed by God to be judge of the living and the dead. To him all the prophets bear witness that everyone who believes in him receives forgiveness of sins through his name” [Acts 10:34-43].
The context is one of the most significant turning-points in the Book of Acts. This text explains not only how Cornelius came to be saved, but how we — the Gentiles — can be saved. This came after Peter had received his vision and heard the voice from heaven declare: “What God has made clean, do not call common” [Acts 10:15].
Peter was commanded to follow three men, who took him to the house of a Roman centurion, Cornelius: “And they said, ‘Cornelius, a centurion, an upright and God-fearing man, who is well spoken of by the whole Jewish nation, was directed by a holy angel to send for you to come to his house and to hear what you have to say’” [Acts 10:22].
Notice carefully that Peter is told that Cornelius was directed by an angel to hear what Peter has to say. When Peter arrives, Cornelius declares to his entire household: “Now therefore we are all here in the presence of God to hear all that you have been commanded by the Lord” [Acts 10:33].
This is one of the most powerful teachings in Scripture about the proper disposition of a congregation. This congregation may have been relatively small, but it was ready to hear a word from the Lord, delivered through God’s preacher.
Just imagine if every congregation awaited every sermon with such an announcement: ”Now therefore we are all here in the presence of God to hear all that you have been commanded by the Lord.”
Imagine the expectation that statement reflects; the faithful eagerness that statement projects. They were gathered to hear and to receive and to believe all that God would command his preacher to say.
So Peter opened his mouth. That is the very next verse — “So Peter opened his mouth and said: ‘Truly I understand that God shows no partiality’” [Acts 10:34]. So Peter opened his mouth. That is the essential act of preaching reduced to five earth-shaking words. So Peter opened his mouth.
In his commentary on Ephesians 6:19-20, Peter O’Brien notes: “The expression ‘to open the mouth’ appears in contexts of solemnity where a grave or important utterance from God is about to be made.”
We can imagine no more important utterance than this — salvation is for the Gentiles, too.
This phrase is used in the Old Testament as well, with reference to prophetic utterance. In Ezekiel 3:2, the prophet says, “So I opened my mouth, and he gave me this scroll to eat.” He was to eat the Word, then preach the Word. In Ezekiel 33:22, Ezekiel explains that he was ready to preach when God opened his mouth, and “so my mouth was opened.”
So Peter opened his mouth. He obeyed the call. He fulfilled his calling. He did not remain silent or hide, he opened his mouth and declared all that God had commanded him to say. Paul once asked the Ephesian Christians to pray “that words may be given to me in opening my mouth boldly” [Ephesians 6:19].
And what did God command him to speak?
That everyone who believes in him receives the forgiveness of sins through his name. Everyone. Jews who believe in him receive the forgiveness of sins through his name. Gentiles who believe in him receive the forgiveness of sins through his name.
Peter had declared the story of Jesus, who went about doing good and healing, but was put to death by hanging him on a tree. God raised him up on the third day, and after appearing to many witnesses he commanded the apostles to preach the gospel to all people, to all nations.
The Christian ministry requires courage, and we can see even more courage required in the near future. There may well be a higher price exacted for opening our mouths. But God has called us to open our mouths so that others can hear his voice, believe, and be saved — so that his church will be fed and taught, and be matured. Can you imagine any higher calling than that?
So, dear preacher, go ye into all the world, and open your mouth.