Sermons will not be marked by authority and power unless they are marked by truth that the Holy Spirit can honour. The word of God is to be explained. That has to be the heart of the sermon. There is a real danger that we become over concerned about such things as delivery, while the New Testament is insistent on the content: "let it be as the words of God" (1 Peter 4:11). The authority of the preaching comes from the text of Scripture. It is God-given power which honours his own word.