In topical preaching, as opposed to expository preaching from the Bible, ministers “take a topic, they appeal to one or two Scriptures to illustrate the topic, they tell stories from their experience and other people’s experience to illustrate the topic,” and all the while the focus of the sermon shifts from the Bible to the preacher. The authority by which a listener is expected to believe what the preacher says “is the human authority of a knowledgeable person,” and nothing more.
Needless to say, Packer is opposed to this procedure of using “the texts … as a convenience for illuminating the topic” instead of the Bible “being expounded as the Word of God.” In topical preaching, according to Packer, preachers do not regard themselves as being “mouthpieces for messages from biblical texts.” When the Bible is not at the center of preaching, “biblical content is made to appear as part of the speaker’s own wisdom,” instead of emerging as the authority for what is said.
What is at stake here is the question of what constitutes the authority for what a preacher says, and that in turn relates to why the person in the pew should regard what the preacher says as being true. When a preacher speaks in his own voice only, the listener has only one option in regard to authority, namely, that the preacher is the authority for what is said. By contrast, when a preacher allows the Bible to provide the primary frame of reference for a sermon, the Bible emerges as the authority and reason for belief. But, of course, the Bible speaks through the mediation of the preacher. Particularly helpful here is Packer’s metaphor that “the activity of preaching the Bible … unlocks the Bible to both mind and heart.”
Ryken, Leland (2015-10-14). J. I. Packer: An Evangelical Life (pp. 362-363). Crossway. Kindle Edition. (Emphasis Mine)