Three quotes from Bryan Chapell, Christ-Centered Preaching, 94:
“As the Spirit of God uses our words to communicate his truth, we speak for God.” (adapted from Westminster Shorter Catechism, 89)
Martin Luther: “Now let me and everyone who speaks the word of Christ freely boast that our mouths are the mouths of Christ. I am certain indeed that my word is not mine, but the word of Christ. So must my mouth be the mouth of him who utters it.”
John Calvin: “Among the many excellent gifts with which God has adorned the human race, it is a singular privilege that he deigns to consecrate to himself the mouths and tongues of men in order that his voice may resound in them.” (Institutes 4.1.5)
Chapell’s conclusion and warning:
This powerful image should caution us never to speak with a tone that compromises Christ’s authority or contradicts his care. We represent him. Therefore we must consider how he would speak were he to address our listeners with the truths committed to our care. If the words we are saying came from Christ’s mouth how would he say them? Our words must reflect his character as well as his truth if our preaching remains true to him.