In his book, On Christian Doctrine, Augustine sets out some of the essential doctrines of the Christian faith and gives guidelines for interpreting Holy Scripture. In the final section of the work he devotes attention to the matter of how Christian doctrine ought to be communicated. Clearly after having painstakingly discussed theological and hermeneutical matters, he does not believe that a man should simply turn up in church and say the first thing that comes into his head. Preparation for preaching involves sound exegesis, deep theological reflection and careful consideration of the applicatory force of biblical truth,
The eloquent divine, then, when he is urging a practical truth, must not only teach so as to give instruction, and please so as to keep up the attention, but he must also sway the mind so as to subdue the will. For if a man be not moved by the force of truth, though it is demonstrated to his own confession, and clothed in beauty of style, nothing remains but to subdue him by the power of eloquence. (St Augustine of Hippo (2012-07-08). On Christian Doctrine (Kindle Locations 4808-4813). Veritatis Splendor Publications. Kindle Edition.)
As he says, 'Accordingly, he who is anxious both to know and to teach should learn all that is to be taught, and acquire such a faculty of speech as is suitable for a divine.' But, Augustine does not yet consider the preacher's preparatory work to be done. He urges that the 'Christian orator' give himself to prayer for the empowering presence of the Spirit in preaching. Through his Spirit God enables the preacher to speak the Word in a way that is most appropriate for the congregation,
And so our Christian orator, while he says what is just, and holy, and good (and he ought never to say anything else), does all he can to be heard with intelligence, with pleasure, and with obedience; and he need not doubt that if he succeed in this object, and so far as he succeeds, he will succeed more by piety in prayer than by gifts of oratory; and so he ought to pray for himself, and for those he is about to address, before he attempts to speak. And when the hour is come that he must speak, he ought, before he opens his mouth, to lift up his thirsty soul to God, to drink in what he is about to pour forth, and to be himself filled with what he is about to distribute. For, as in regard to every matter of faith and love there are many things that may be said, and many ways of saying them, who knows what it is expedient at a given moment for us to say, or to be heard saying, except God who knows the hearts of all? And who can make us say what we ought, and in the way we ought, except Him in whose hand both we and our speeches are? Accordingly, he who is anxious both to know and to teach should learn all that is to be taught, and acquire such a faculty of speech as is suitable for a divine. But when the hour for speech arrives, let him reflect upon that saying of our Lord's as better suited to the wants of a pious mind: "Take no thought how or what ye shall speak; for it shall be given you in that same hour what ye shall speak. For it is not ye that speak, but the Spirit of your Father which speaketh in you." The Holy Spirit, then, speaks thus in those who for Christ's sake are delivered to the persecutors; why not also in those who deliver Christ's message to those who are willing to learn? St Augustine of Hippo (2012-07-08). On Christian Doctrine (Kindle Locations 4864-4888). Veritatis Splendor Publications. Kindle Edition.
As envisaged by Augustine, preaching involves both diligent preparation and a readiness to improvise when in the act of declaring the Word under the guidance of the Holy Spirit. That is a fine line to tread. If a speaker is too tied to his sermon notes, his message will take the form of a lecture. His delivery will lack the spontaneity that comes from the interplay of preacher and congregation that is of the essence of preaching. On the other hand, if the preacher presumes to speak without due preparation, trusting in the impulse of the moment under the guise of relying on the Spirit, he is a lazy slacker who has neglected his calling to 'labour in the word and teaching' (1 Timothy 5:17). The preacher must give himself to 'prayer and the ministry of the word' (Acts 6:4). As Augustine put it most eloquently, 'And when the hour is come that he must speak, he ought, before he opens his mouth, to lift up his thirsty soul to God, to drink in what he is about to pour forth, and to be himself filled with what he is about to distribute.'