Paul has just been speaking about the human conscience bearing witness with God’s Law (v. 15). Now he applies the Law to the conscience to bring “the knowledge of sin.” He personalizes the Eighth Commandment: “You who preach that a man should not steal, do you steal?” (v. 21). The conscience agrees with the Commandment that it is morally wrong to steal. This is why I find it so beneficial to ask someone who professes to be a “good” person how many lies he thinks he has told in his life. When he says, “Hundreds,” I ask, “What do you call someone who tells lies?” When he answers, “A liar,” he is being shown the personal nature of his sins. Then in v. 22, Paul uses the Seventh Commandment, personalizing it by asking, “Do you commit adultery?” When you take someone through the moral Law, on this Commandment don’t forget to explain that Jesus said if we even look with lust, we commit adultery in our hearts (see Matt. 5:27,28).
Dr. J Gresham Machen said, “A new and more powerful proclamation of [the] Law is perhaps the most pressing need of the hour; men would have little difficulty with the gospel if they had only learned the lesson of the Law.”
Romans 2:21 You, therefore, who teach another, do you not teach yourself? You who preach that a man should not steal, do you steal?