We have lost the connection between Law and Gospel. We think we can make Christians into better people without the Gospel, and we think we can make unbelievers into believers without mentioning the Law.
I often hear sermons-even intentionally evangelistic sermons-where people are urged to "accept Jesus as your Lord and personal Savior." But they haven't been told why they need him, and they haven't been told what he's done for them actively (in his righteous life) or passively (in his atoning death). There isn't any Gospel here in many of these sermons, and there isn't any compelling Law either.
The Reformation talked about three uses of the Law. The first is the civil use: God gives us moral law to have a society so we can have a culture where we don't just tear each other apart. The third use (skipping the second one for a moment) is as a guide to show Christians how we should live, to help us see what is a God-pleasing lifestyle. The first use and the third use of the Law are worthy of study. I would suggest, though, that for a sermon what you need to be doing primarily is to apply the second, or evangelical, use of the Law. Preach the Law so that it creates repentance, so that people wake up to their need of Christ.
People tend to be self-righteous, and that's the big obstacle to the Gospel. "I'm OK, I don't need anyone, I haven't done anything wrong anyway." That's the attitude that needs to be crushed and slain, and it is slain when people hear the rigor of the Law and how desperate their condition is as they stand before a holy God. Once they realize that, then they can hear the message of free forgiveness in Christ, of his atonement, of how he paid the penalty and grants us his perfection. Only then does that become good news that people seize on with life-changing faith.
Our tendency a lot of times is to soften the Law, to make it something that we can fulfill. Preachers mustn't make the Law sound easy to fulfill. They need to make people realize that they are not fulfilling the Law. If a sermon just extracts principles from the Bible that people are already doing, or that they can do with a few little adjustments, you're not really preaching the Law in this sense. What you're doing is preaching moralism, and a lot of sermons and Sunday school materials are just moralism. They imply that you can live a good life, and that if you just do the right things, then God will be happy with you, and you'll have a happy middle-class lifestyle, and everything will go fine and you're really a good person. The irony is that if people are left in moralism, you're leaving them in their sins; you're creating complacency, and worse, you're creating self-righteousness. Yet it's self-righteousness that is the obstacle to the Gospel, the obstacle to justification, and it's the obstacle to sanctification. Only the Law can awaken us to our need of the Gospel. And only the Gospel can ever give us the genuine desire to grow in holiness.
