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: Really take time to look at this verse
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.