"... what else should be done if we hope to see conversions? Assuredly we should be careful to preach most prominently those truths which are likely to lead to this end. What truths are those? I answer we should first and foremost preach Christ and him crucified. Where Jesus is exalted souls are attracted – "I, if I be lifted up will draw all men unto me." The preaching of the cross is to them that are saved the wisdom of God and the power of God. The Christian minister should preach all the truths which cluster around the person and work of the Lord Jesus, and hence he must declare very earnestly and pointedly the evil of sin, which created the need of a Saviour. Let him show that sin is a breach of the law, that it necessitates punishment, and that the wrath of God is revealed against it. Let him never treat sin as though it were a trifle or a misfortune, but let him set it forth as exceeding sinful. Let him go into particulars, not superficially glancing at evil in the gross, mentioning various sins in detail, especially those most current at the time: such as that all-devouring hydra of drunkenness, which devastates our land; lying, which in the form of slander abounds on all sides; and licentiousness, which must be mentioned with holy delicacy, and yet needs to be denounced unsparingly. We must especially reprove those evils into which our hearts have fallen, or are likely to fall. Explain the ten commandments and obey the divine injunction: "Show my people their transgressions, and the house of Jacob their sins." Open up the spirituality of the law as our Lord did, and show how it is broken by evil thoughts, intentions, and imaginations. By this means many sinners will be pricked in their hearts. Old Robbie Flockhart used to say, "It is of no use trying to sew with the silken thread of the gospel unless we pierce a way for it with the sharp needle of the law." The law goes first, like the needle, and draws the gospel thread after it: therefore preach concerning sin, righteousness, and judgement to come. Let such language as that of the fifty first Psalm be explained: show that God requireth truth in the inward parts, and that purging with sacrificial blood is absolutely needful. Aim at the heart. Probe the wound and touch the very quick of the soul. Spare not the sterner themes, for men must be wounded before they can be healed, and slain before they can be made alive. No man will ever put on the robe of Christ's righteousness till he is stripped of his fig leaves, nor will he wash in the fount of mercy till he perceives his filthiness. Therefore, my brethren, we must not cease to declare the law, its demands, its threatenings, and the sinner's multiplied breaches of it."
Charles Haddon Spurgeon, Lectures to My Students, 337-338.