Gravity and earnestness in preaching is appropriate not only (as we have seen) because preaching is God's instrument for the weighty business of saving sinners and reviving his church, but also because it is God's instrument for preserving the saints. Paul says in 2 Timothy 2:10, "I endure everything for the sake of the elect, that they also may obtain the salvation which in Christ Jesus goes with eternal glory." Labor on behalf of the elect, therefore, is not icing on the cake of their eternal security. It is God's appointed means of keeping them secure. Eternal security is a community project (Heb. 3:12-13) and preaching is part of God's securing power. He calls effectually by the Word and he keeps effectually by the Word.
We can say that eternal security is certain for the Christian, yet avoid a mechanical view that drains the blood-earnestness right out of the weekly ministry of preaching to the saints. Biblically God uses the earnest application of the means of grace to hold his people secure; one of those means is the preaching of God's Word. Heaven and hell are at stake every Sunday morning not merely because unbelievers might be present, but also because our people are saved "IF they continue in the faith" (Col. 1:23). Paul connects the steadfastness of faith with the preaching of the Word of God in the gospel (Rom. 10:17). (pp. 58-59)These thoughts take me to another portion of Piper's book:
How utterly dependent we are on the Holy Spirit in the work of preaching! All genuine preaching is rooted in a feeling of desperation. You wake up on Sunday morning and you can smell the smoke of hell on one side and feel the crisp breezes of heaven on the other. You go to your study and look down at your pitiful manuscript, and you kneel down and cry, "God, this is so weak! Who do I think I am? What audacity to think that in three hours my words will be the odor of death to death and the fragrance of life to life (2 Cor. 2:16). My God, who is sufficient for these things?" (pp. 37-38)