Preaching the reality of Hell, without using the Law to bring the knowledge of sin (the Ten Commandments), can do a great deal of damage to the cause of the gospel.
A sinner cannot conceive of the thought that God would send anyone to Hell, as long as he is deceived into thinking that God’s standard of righteousness is the same as his. Paul “reasoned” with Felix regarding righteousness, temperance, and judgment to come (Acts 24:25).
This is the righteousness that is of the Law and judgment by the Law. Felix “trembled” because he suddenly understood that his intemperance made him a guilty sinner in the sight of a holy God. The reality of Hell suddenly became reasonable to him when the Law was used to bring the knowledge of sin.
Imagine if the police burst into your home, arrested you, and shouted, “You are going away for a long time!” Such conduct would probably leave you bewildered and angry. What they have done seems unreasonable.
However, imagine if the law burst into your home and instead told you specifically why you were in trouble: “We have discovered 10,000 marijuana plants growing in your backyard. You are going away for a long time!” At least then you would understand why you are in trouble. Knowledge of the law you have transgressed furnished you with that understanding. It makes judgment reasonable. Hell-fire preaching without use of the Law to show the sinner why God is angry with him will more than likely leave him bewildered and angry, for what he considers unreasonable punishment.