A pithy summary from R. de la Broise, Bossuet and the Bible, 1890, pp. 160-1:
“To preach the word of God, to go hear the word of God,” these are the expressive words of the Christian language. They neatly outline one of the distinct characteristics of preaching, one of the points which make the genre absolutely proper to Christianity, and nothing else in antiquity corresponds to it. In the Christian Church, the Bible is the “word of God,” and the preacher is only its herald and interpreter; his first duty is to know it well, to also know well the most established commentaries, and to transmit it without alteration or corruption; his originality, to distribute it a propos, to take from this bottomless treasure that which meets the circumstances and the hearer, to make heard from this divine word justly what is necessary, and from there appropriate applications.