Preaching involves both law and gospel, not as stale categories to distinguish commands from promises but as a stripping bare of the truth about the human condition and a revealing of the truth about God's gracious heart. Luther, reflecting on the dilemma of preaching, once complained that when he preached the unconditional grace of God in the gospel, the lazy and licentious always took it as an opportunity to remain in their sin. When he preached the law, the weak despaired. Either way, preaching seemed to miss its goal. However, Luther added, for the sake of the weak he would proclaim the gospel, since those who use its forgiveness as an excuse to sin would manipulate everything to their advantage anyway.
That's wise on so many levels I hardly know where to begin.