Often times Christian become confused about what the purpose of God's law is 1) in general and 2) for believers. They tend to go to two extremes 1) legalism and 2) antinomianism. John Calvin saw in God's Word 3 purposes for God's law. Here they are as presented in Joel Beeke's book Overcoming the World.
Let us let the law do what it is meant to do according to Scripture. Then we can truly say with the Psalmist that God's Law is more desirable to us than fine gold and sweeter to our taste than the drippings of the honeycomb. (Psalm 19:10)
- It restrains sin and promotes righteousness in the church and society, preventing both from lapsing into chaos.
- It disciplines, educates, convicts, and drives us outside of ourselves to Jesus Christ, the fulfiller and end of the law. The law cannot lead us to a saving knowledge of God in Christ. Rather, the Holy Spirit uses the law as a mirror to show us our guilt, to shut us off from hope, and to bring us to repentance. It drives us to the spiritual need out of which faith in Christ is born. This convicting use of the law is critical for the believer's piety, for it prevents the ungoldly self-righteousness that is prone to reassert itself even in the holiest of saints.
- It becomes the rule of life for the believer. "What is the rule of life which [God] has given us?" Calvin asks in the Genevan Catechism. The answer: "His law." Later, Calvin says the law "shows the mark at which we ought to aim, the goal towards which we ought to press, that each of us, according to the measure of grace bestowed upon him, may endeavor to frame his life according to the highest rectitude, and, by constant study, continually advance more and more.