R.C. Sproul on the threefold use of the law.
The first purpose of the law is to be a mirror. On the one hand, the law of God reflects and mirrors the perfect righteousness of God. The law tells us much about who God is. Perhaps more important, the law illumines human sinfulness. Augustine wrote, “The law orders, that we, after attempting to do what is ordered, and so feeling our weakness under the law, may learn to implore the help of grace.” The law highlights our weakness so that we might seek the strength found in Christ. Here the law acts as a severe schoolmaster who drives us to Christ.
A second purpose for the law is the restraint of evil. The law, in and of itself, cannot change human hearts. It can, however, serve to protect the righteous from the unjust. Calvin says this purpose is “by means of its fearful denunciations and the consequent dread of punishment, to curb those who, unless forced, have no regard for rectitude and justice.” The law allows for a limited measure of justice on this earth, until the last judgment is realized.
The third purpose of the law is to reveal what is pleasing to God. As born-again children of God, the law enlightens us as to what is pleasing to our Father, whom we seek to serve. The Christian delights in the law as God Himself delights in it. Jesus said, “If you love Me, keep My commandments” (John 14:15). This is the highest function of the law, to serve as an instrument for the people of God to give Him honor and glory.