In this series John Gerstner is going through church history quite briefly. I recommend the series for anyone just starting in church history (there are better resources for the advanced learner). Here he is talking about Augustine and his response to the controversies of his day. For a majority of the lecture prophecy has no part in the lecture. But towards the end of the video he touches on the topic of sacraments being voided by faithless teachers.
Augustine was quite clear that a bad minister did not ruin the sacrament. And that includes preaching! Preaching a sacrament? In a way yes. Gerstner drives home the link between prophecy and preaching at a most crucial verse. I think it stands as a stern warning against all modern preachers. Many in that day may say "Lord, Lord".
Enjoy the lesson here. For those only wanting to hear the prophecy portion you can start the video at the 21:50 mark. Enjoy!