Preaching is a strange business. It requires you to stand up and speak with authority and pointed passion to people who may well be your intellectual and spiritual betters. But a man who has confidence in the word of God has everything he needs for the task. Here’s how Herman Bavinck put it:
For if a minister is not convinced of the divine truth of the word he preaches, his preaching loses all authority, influence, and power. If he is not able to bring a message from God, who then gives him the right to act on behalf of people of like nature with himself? Who gives him the freedom to put himself in the pulpit [a few feet] above them, to speak to them about the highest interest of their soul and life and even to proclaim to them their eternal weal or woe? Who would dare, who would be able to do this, unless he has a word of God to proclaim?
-- Herman Bavinck, Reformed Dogmatics, vol. 1 (p. 461)