“For teaching, of course, true eloquence consists, not in making people like what they disliked, nor in making them do what they shrank from, but in making clear what was obscure; yet if this be done without grace of style, the benefit does not extend beyond the few eager students who are anxious to know whatever is to be learnt, however rude and unpolished the form in which it is put… Accordingly a great orator [Cicero] has truly said that ‘an eloquent man must speak so as to teach, to delight, and to persuade.’ Then he adds: ‘To teach is a necessity, to delight is a beauty, to persuade is a triumph.’…
“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.”
Augustine, On Christian Doctrine, Book IV: 11, 12, 15.