The charge of the pastor is to feed the people of God. This means that he must feed them what people of God need for sustenance, and this is the word of God. Likewise, it is the word of God which is powerful, living and active. This is what God's people, the church, hungers for. Therefore, it is the pastor's responsibility to teach the word of God to the people of God.
The charge of the pastor comes from God. This means that the pastor speaks for God, and thus it is his responsibility to bring the word of God, and not his own word, to the people of God. This means that he is to be under the authority of God in all that he says. He is not to rely on his own understanding, but on the word of God.
Preaching from the Scriptures, then, requires sitting under the authority of God. No preaching is truly the preaching of God's word unless it is restrained by the bounds of Scripture. No preacher, indeed, no one at all, has authority to go beyond the word of God, and this means that no preacher has authority to go beyond what is in the Scriptures when he preaches to the people of God.
This means that the preaching must be from the word of God. It must be from the word of God because that is the only source of nourishment for the people of God, and it must be from the word of God because that is the only source of authority on which the preacher can legitimately stand when he speaks as preacher to the people of God. This means that the preaching must not be from some alien source, that is, it may not come from outside the word of God. This means that the preacher must approach the Scriptures themselves to discover what aim he is to have in each sermon. Any alien motive is denied, because any alien motive is illegitimate. Insofar as an aim or an outline or a message for a sermon does not come from the Scriptures themselves it is like dirt which muddies the living water meant for God's people.
This is not to deny that good historical, linguistic, philosophical, theological or other scholarship is useless to the preacher, but that it is subordinate to the word of God in preaching. But it is to affirm that all scripture is sufficient for life and godliness. It is to deny that what comes to us from outside of Scripture can be used to impose on Scripture a structure which is alien to Scripture. It is to deny preaching on the basis of what is not said in the Scriptures, but is merely hypothesized. We must be careful with God's word, respecting it as God's. We must not be careless, but examine the words of God carefully, to see what he has given us to say. We rely not on human reason or experience or tradition, but on divine revelation, given to us in Scripture. What we preach is not of human ingenuity, but of God's wisdom.