"3. Another thing required for this is, experience of the power of the things we preach to others. I think, truly, that no man preaches a sermon well to others who does not first preach it to his own heart. If someone does not feed on, digest, and thrive by what he prepares for his people, he may give them poison as far as he knows; for unless he finds the power of it in his own heart, he cannot have any ground of confidence that it will have power in the hearts of others. It is an easier thing to bring our heads to preach than our hearts to preach. To bring our heads to preach, is nothing more than to fill our minds and memories with some notions of truth, of our own or other men, and speak them out to give satisfaction to ourselves and others: this is very easy. But to bring our hearts to preach, is to be transformed into the power of these truths; or to find the power of them, both before (in preparing our minds and hearts), and in delivering them so that we may benefit; and to be presented with zeal for God and compassion to the souls of men. A man may preach every day in the week, and not have his heart engaged once. This has lost us powerful preaching in the world and set up quaint orations instead; for such men never seek the experience of it in their own hearts. And so it has come to pass that some men’s preaching, and some men’s not preaching, have lost us the power of what we call the ministry. Though there be twenty or thirty thousand preachers, the nation perishes for lack of knowledge, and it is overwhelmed by all kinds of sins, and it is not delivered from them to this day."
Excerpt from The Duty of a Pastor (1682) - John Owen