I think, not only the content of preaching, but the order of the content is important; indispensable even. J. Gresham Machen, in Christianity and Liberalism, wrote,
The consciousness of sin was formerly the starting-point of all preaching, but today it is gone… Christianity is the religion of the broken heart…. it begins with the consciousness of sin. Without the consciousness of sin, the whole gospel will seem to be an idle tale. (pp. 56-57)
How often is the first use of the Law employed in a sermon to impress upon believers an awareness of God’s holiness and His just demands contrasted with their very real sin? Machen, it seems, would say that if the gospel is to have its intended effect as Good News, then it is necessary for a consciousness of sin to precede that of free mercy in Christ Jesus offered to undeserving sinners.
Likewise if the third use of the law (admonishing and directing grateful believers in a righteous direction for living) is preached without first the message of the Law, followed by that of the gospel—Jesus as the One who died and bore the penalty of our falling short, raised from the dead for our justification (his righteous obedience credited to us)—then, rather than directing and guiding, it takes on a subtle, foreboding tone of demand for obedience in order to be accepted by God.
This can occur, unfortunately, when a pastor assumes the Law as well as the Gospel in his sermons. He may implicitly mention them, but not explicitly as the two words of God’s redemptive story. We already know the information that we’re sinners and that Jesus died for sinners! So sermons, bypassing Law and Gospel, are too often reduced to biblical admonitions sprinkled with promises of God’s grace and faithfulness. The intention is to encourage and exhort believers as how to live in a manner worthy of their Lord. And yet the result too often received is sermon-exhortation as a soft law-demand.