The preacher is a herald; his work is heralding the King's message. . . . Now the herald does not invent his message; he merely transmits and explains it. It is not his to criticise its wisdom or fitness; this belongs to his sovereign alone. On the one hand, . . . he is an intelligent medium of communication with the king's enemies; he has brains as well as a tongue; and he is expected so to deliver and explain his master's mind, that the other party shall receive not only the mechanical sounds, but the true meaning of the message. On the other hand, it wholly transcends his office to presume to correct the tenour of the propositions he conveys, by either additions or change. These are the words of God's commission to an ancient preacher: "Arise, go unto Ninevah, that great city, and preach unto it the preaching that I bid thee."
The preacher's task may be correctly explained as that of (instrumentally) forming the image of Christ upon the souls of men. The plastic substance is the human heart. The die which is provided for the workman is the revealed Word; and the impression to be formed is the divine image of knowledge and true holiness. God, who made the soul, and therefore knows it, made the die. He obviously knew best how to shape it, in order to produce the imprint he desired. Now the workman's business is not to criticise, recarve, or erase anything in the die which was committed to him; but simply to press it down faithfully upon the substance to be impressed, observing the conditions of the work assigned him in his instructions. In this view, how plain is it, that preaching should be simply representative of Bible truths, and in Bible proportions! The preacher's business is to take what is given him in the Scriptures, as it is given to him, and to endeavour to imprint it on the souls of men. All else is God's work. The die is just such, so large, so sharp, so hard, and has just such an "image and superscription" on it, as God would have. Thus He judged, in giving it to us. With this, "the man of God is perfect, thoroughly furnished unto all good works." (2 Tim 3:17) This is enough for us.
Source: R.L. Dabney, Evangelical Eloquence: A Course of Lectures on Preaching, 36-3