Continuing to consider reasons why preaching should be done expositionally, we turn now to the doctrine of the sufficiency of scripture. Protestants have always affirmed that, "The Holy Scripture containeth all things necessary to salvation; so that whatsoever is not read therein, nor may be proved thereby, is not to be required of any man that it should be believed as an article of faith, or be thought requisite or necessary to salvation." This is excerpted from Article V of the Articles of Religion which John Wesley adapted from the 39 Articles for the people called Methodists, and other Protestant confessions articulate similar understandings of this doctrine. The point that the article makes is that everything necessary for salvation has been revealed and preserved in the scriptures. Nothing exists outside the scriptures which must be believed in order to obtain eternal happiness and salvation. The Bible has everything we need to know God and experience his salvation to the utmost.
Neither is this doctrine foisted upon scripture from the outside. 2 Peter 1:3 declares, "His divine power has given us everything needed for life and godliness, through the knowledge of him who called us by his own glory and goodness" (NRSV). That is, we have been given everything we need not only to experience birth into the new life that God has for us, but also to experience the the power of salvation to cleanse us from the power of sin in order that the character of God may be manifest in godliness in all aspects of life. The question is: how do we receive such blessing? Peter's answer is that we receive it through the knowledge of God and of Jesus our Lord (cf. 1:1-2). And where do obtain this knowledge. God has revealed himself and preserved that revelation in the writings of the Old and New Testaments. Therefore, the scriptures are sufficient to provide everything we need for the salvation that consists in eternal life and living in holiness.
By the way, it's no small thing that at the end of the letter, Peter compares Paul's letters to the "other scriptures," by which he means the Hebrew scriptures (3:16). By the early second half of the first century, the writings of the Apostles were considered to have equal authority as the Old Testament.
In light of these considerations, what can we say about preaching? Well, if the Bible contains everything necessary to life and godliness and is entirely sufficient for salvation, then the preacher ought to do the best he can to make sure that his congregation is exposed to the scriptures and the scriptures are exposed to his congregation. This is the task of expositional preaching: to expose the scriptures by explaining them clearly and applying them to the life of the church. Thus, the systematic exposition of the biblical text is the most important thing a preacher can do in ministry to his people. He serves them best by serving them the Word.
The preacher who takes it as his aim to do otherwise necessarily undermines the sufficiency of the scriptures for salvation. By introducing biblically unfounded reflection or the latest self-help psychologizing, the preacher either says that there are other things out there that can lead to life and godliness or says that he doesn't much care whether his congregation receives what they need for life and godliness.
Why do I preach expositionally? Because the scriptures are sufficient to provide everything we need to know God and the fullness of his salvation. Expositional preaching is the best way, if not the only way, to make sure that our people receive the Word of life which they so desperately need.