Incorporating the Law into the gospel presentation does many things. It primarily shows the sinner that he is a criminal, and that God is his judge. The Law (in the hand of the Holy Spirit) stops his mouth and leaves him guilty before God (see Romans 3:19-20). It reveals that he deserves nothing but judgement for his crimes. Like a faithful prosecutor, it points its accusing finger, and so the sinner's stirred conscience bears witness and also points its finger at the criminal (see Romans 2:15). The verdict is 'guilty,' and the condemnation is just.
This is the scenario that I try and paint for the sinner. I do my best to put him in the courtroom on the Day of Judgement, with the hope that he will understand the mercy that God offers him in Christ.
For years when I have done this, I have then said, 'You broke God's Law, and Jesus paid your fine in his life's blood.' But, early in 2008, I added the words, 'It was a legal transaction. You broke God's Law (the Ten Commandments), and Jesus paid your fine. That means that God can legally dismiss your case. You can leave the courtroom on the Day of Judgement because another paid your fine. Does that make sense?'
From the first time I said those words, I noticed again and again, light go on in the eyes of my hearers. While this is certainly not a magic formula, many suddenly understood what I was trying to say when I explained the gospel that way. I can't point to a Bible verse that uses this exact language, but I can say that legality is the essence of the cross. It was God's love for justice and for guilty sinners, that drove him to Calvary.
Man is unique among God's creation. He is forensic by nature. He intuitively understands the principles of law, retribution and mercy, because he is made in the image of God. That's why every civilization sets up court systems. That's why there is a resonance with a sinner (see Romans 2:12-16). So when Paul then uses the Law to bring the knowledge of sin, he knows that he will find reverberation in their hearts (see Romans 2:21-24).
God is the 'habitation of Justice' (see Jeremiah 21:33). We are guilty criminals. The fine has been paid, and we can leave the courtroom. So carefully explaining the gospel message, using legal vernacular to those whose understanding is 'darkened,' gives new light on what they before perceived to be just an old and irrelevant story. It's as though they suddenly say, 'So that's what the cross was about!' And that's what we want them to know.