Preaching that is full of advice about how people can live like Christians can’t transform, only preaching that points people to God’s transforming power in the person and work of Jesus Christ has the hope of change.
The conclusion of an article about Luther’s theology by Phillip Cary at First Things.
How we have always been justified by faith alone is best seen in light of Luther’s distinction between law and Gospel. Both the law of God and the Gospel of Christ are God’s word, but the former only gives us instructions while the latter gives us Christ. For the law tells us what to do, but the Gospel tells us what Christ does. The distinction grows out of Augustine’s insistence, in his great treatise On the Spirit and the Letter, that telling us to obey the law of love does not help us do it from the depths of our hearts; only the grace of Christ can give us such a heart. Luther merely adds: The place to find the grace of Christ is in the Gospel of Christ.
A great many preachers, Protestant as well as Catholic, overlook the distinction between law and Gospel, thinking they can change people’s lives by giving them practical advice—as if telling them how to be inwardly transformed could help them do it. Augustine already knew better. Luther’s addition to Augustine’s insight is merely the glad recognition that there is indeed something preachers can do to help us be transformed: Instead of advice, they can give us Christ.