As to purpose... One way I think about purpose is to think in terms of direction. And with that in view my question is - when it comes to a sermon - where is the preacher heading or what is he hoping to accomplish? In other words, to what destination is he hoping to bring his hearers? A preacher always has a purpose that guides his sermon, even if it is one of which he isn't particularly conscious. He is indeed intending to communicate something with his words to his listeners, even if that intention and something is isn't always evident to himself.
As image bearers of God, what we do and say always has a purpose or intention. We were created with a will, a will by which we set goals or purposes and choose to do this or that to accomplish those goals. And that purposing always sets a direction toward an end. In other words, the why one says or does something always determines the what one says or does and so is central in determining the final destination of the where one ends up.
For instance, if you know that your house guest is hungry will you go to the clothes closet to find food with which to feed him? Of course not. You'll go to the kitchen and most probably the refrigerator. And the reason is obvious. Because you know that's where food is kept and you know going to the kitchen is the best route to take in order to satisfy your guest's hunger. Therefore based on what you know you set a purpose or direction toward the kitchen. You carry out that purpose, go to the kitchen and get the food. You then serve it to him and he eats.
Can it be any less the case when it comes to feeding the flock of God? The preacher needs to know not only that his people are hungry for the food of heaven. He needs to know what the food of heaven is (doctrinal information and truths?), where to find it (duties and admonitions to faithfulness?), and how one serves it.
The analogy breaks down at this point because Christians are sinners and are always, to some degree, in denial as to their hunger/need for the heavenly. Ironically, they're often inclined to avoid that which they truly need and go after that which they shouldn't - an operational description of what it means to be a sinner. Unlike someone who is physically hungry we sinners aren't necessarily convinced of our spiritual hunger and need. What we tend to be convinced of is that we're getting along "fairly well" and that by just a little more faithful living we can satisfy our hunger for that "I'm really am OK" feeling. Or maybe we fall into the camp of those who are convinced that there is something wrong with them. We don't know why that is. But we know it disqualifies us from the assurance that the blessings of the Christian life are really ours. Too aware of our failures, we are resigned to our plight that we don't measure up. We believe in Christ, yet are convinced that, unlike others, we just can't produce enough sanctified living to warrant peace with God. Saved or not, sinners don't readily see that their failed strategies for pursuing a satisfying life or those that gloss over their failures in order to avoid pain are nothing more than broken cisterns. It seems natural to continue wandering along our path.
This brings into focus the core of the problem to be addressed by the preacher. What sinners have is less of a hunger and more of a critical need. Yet it's a need to be cured of something we don't naturally view as an ill. So our natural rejection of God's diagnosis needs addressing, and not just once. So how is a sinner convinced of his need?
Persuasion and preaching... We saints of God are in need of being persuaded that we are sinners, persuaded not once but regularly. Why? Because part of being a sinner is that we are prone to unbelief and are inclined to reject the idea that we're really all that bad or sinful. If we feel that our sin isn't all that deadly and condemnable then we'll not feel much of a need for the Savior's cure. We'll remain somewhat convinced that we can "with God's grace" pull things off. So sermons ought to be about persuading sinners/saints of their sin that they may again fall on their knees before the cross and trust in Christ, coming to him in weakness and repentance. Preachers need to be persuaders.
But the message of the good news of Jesus Christ declares that God's judgment and curse have fallen upon his Son in whom, according to God's redemptive plan, is the forgiveness of sins received through faith for all who are called to him. Apart from Christ crucified there is no living the Christian life. As J. Gresham Mach wrote, "For clearly if Christianity is anything it is a way of getting rid of sin" (Christianity and Liberalism, p. 90). And as the apostle Paul wrote near the end of his life, It is a trustworthy statement, deserving full acceptance, that Christ Jesus came into the world to save sinners, among whom I am foremost of all (1 Tim. 1:15).
So deeply rooted in our hearts is unbelief, so prone are we to it, that while all confess with the lips that God is faithful, no man ever believes it without an arduous struggle...
But that the foretaste which we obtain from any minute portion of faith is certain, and by no means fallacious, he elsewhere shows, when he affirms that "We all, with open face beholding as in a glass the glory of the Lord, are changed into the same image, from glory to glory, even as by the Spirit of the Lord," (2 Corinthians 3:18.) In such degrees of ignorance much doubt and trembling is necessarily implied, especially seeing that our heart is by its own natural bias prone to unbelief. To this we must add the temptations which, various in kind and infinite in number, are ever and anon violently assailing us. In particular, conscience itself, burdened with an incumbent load of sins, at one time complains and groans, at another accuses itself; at one time murmurs in secret, at another openly rebels. Therefore, whether adverse circumstances betoken the wrath of God, or conscience finds the subject and matter within itself, unbelief thence draws weapons and engines to put faith to flight, the aim of all its efforts being to make us think that God is adverse and hostile to us, and thus, instead of hoping for any assistance from him, to make us dread him as a deadly foe. (Institutes)
Jesus offered himself as the true food and the true drink to be received by sinners through faith and not as just a one-time conversion experience or infrequent meal, but for daily sustenance and refuge from the accusations our consciences continually bring to mind. Jesus said "I am the bread of heaven and the water that satisfies." Faith that looks to him feeds on his death and resurrection. He is the Lamb of God who takes away sin with its always heavy load of guilt.
Do Christ's sheep still sin? Do they sometime struggle under the burden that their sins make them second class Christians, insecure under a threat of condemnation? It's the grace of the gospel - the message of Christ's shed blood - that daily cleanses guilty consciences and is the food that nourishes and maintains the faith of the saints. And that message is true food to be proclaimed and served to ears that hear which message then goes directly to hearts that believe.
I beseech you to be persuaded that here you are to work nothing, here you are to do nothing, here you are to render nothing unto God, but only to receive the treasure, which is Jesus Christ, and apprehend him in your heart by faith, although you be never so great a sinner; and so shall you obtain forgiveness of sins, righteousness, and eternal happiness; not as an agent but as a patient, not by doing, but by receiving. Nothing here comes betwixt but faith only, apprehending Christ in the promise. this, then, is perfect righteousness, to hear nothing, to know nothing, to do nothing of the law of works; but only to know and believe that Jesus Christ is now gone to the Father, and sitteth at his right hand, not as a judge, but is made unto you of God, wisdom, righteousness, sanctification, and redemption. (Marrow of Modern divinity, Edward Fisher)... and the synagogue having been dismissed, many of the Jews and of the devout proselytes did follow Paul and Barnabas, who, speaking to them, were persuading them to remain in the grace of God... and he was reasoning in the synagogue every sabbath, persuading both Jews and Greeks. (Acts 13:43; 18:4)