Here is Christopher Ash’s summary of the authority of the preacher.
A. The preacher exercises the authority of Christ in the Church
1. Under the old covenant, God governed his people by the written word preached by prophets.
(a) The primacy of the prophet in the people of God.
(b) The authority of God is exercised not by the written word, but by the written word preached.
2. Under the new covenant, Christ governs his people by the written word preached by preachers.
(a) The progressive revelatory role of the prophets is fulfilled in Christ the Prophet.
(b) But Christ the Prophet is no longer here!
(c) And so we need preachers to continue the proclamatory work of the prophets.
B. The preacher’s authority is a borrowed authority, only obtained by much toil, sweat and with deep humbling.