What is preaching? It is proclamation, not just moralizing. It is Good News, not just good advice; it is gospel, not just law. Supremely, it is about God and what he has done, not just about us and about what we ought to do.
At the heart of it, preaching is the telling and retelling of Christ's story and our stories from creation to parousia. It is the remembering of the stories with a special kind of remembrance of which the Bible speaks (Old Testament zakar, New Testament anamnesis), a remembrance that does not merely call to mind the thing remembered, but that makes it real, present, potent, and demanding here and now.
To be biblical a sermon must be rooted in the particularities of a passage of Scripture, in the immediate and wider context of the passage and in the cosmic sweep of the Christian gospel as a whole. Only thus can I be sure that I have heard an authentic proclamation of the Word--not just that I have admired the silver-tongued eloquence of the King's herald or even the splendor of his robes, but that I have caught a fresh glimpse of the face of the King himself. The preacher cannot guarantee that this will happen. The preaching event, that moment of royal meeting, is something we cannot create or command. Only God can do that.