In the Old Testament Scripture, we see God give His law to Israel, His chosen people. God required them to do what is right by obeying His laws because, as their sovereign ruler, God had the right to command their obedience. He also had the power and right to punish their disobedience when they violated His law—which the Bible defines as sin. Sin is breaking God's law.
As we look at God's laws in the Old Testament, we see that some laws are temporary conditions which only God has the authority to annul, while other laws are permanent and eternal for all people. The dietary and ceremonial laws of Israel were temporary laws for their society, showing them to be a people set apart for God to the surrounding nations. The moral laws of God are eternal, and God commands us to live according to His moral law which is found in the Ten Commandments.
Perhaps you have asked the questions: “What is the purpose of the Old Testament law in my life? Is the Old Testament law irrelevant, or am I still obligated to keep it? And if it is relevant, must I keep all of it or just certain portions of it?”
During the Reformation of the 16th Century, the Reformers clearly taught from Scripture that salvation is by grace alone, not through the keeping of the law. However, the Reformers never laid aside God's moral law. John Calvin wrote in his Institutes about the "Threefold Use of the Law" to show the importance of the law in the Christian's life.
First, he said, the law acts as a mirror reflecting to us both the perfect righteousness of God and our own sinfulness and shortcomings. The law shows us what God is like, revealing to us the perfect moral character of God. At the same time, we look into the mirror of God's law and it clearly reflects our sinfulness. Paul tells us that it is only through God's law that we have the knowledge of sin:
For by works of the law no human being will be justified in his sight, since through the law comes knowledge of sin. (Romans 3:20)
What then shall we say? That the law is sin? By no means! Yet if it had not been for the law, I would not have known sin. For I would not have known what it is to covet if the law had not said, "You shall not covet." But sin, seizing an opportunity through the commandment, produced in me all kinds of covetousness. For apart from the law, sin lies dead. I was once alive apart from the law, but when the commandment came, sin came alive and I died. The very commandment that promised life proved to be death to me. For sin, seizing an opportunity through the commandment, deceived me and through it killed me. (Romans 7:7-11)
As the law reveals to us our sin, we see clearly our guilty condition, condemnation, and our great need for a pardon which leads us to repentance and faith to Christ (Gal. 3:19-24). The law is used as a school master to drive us to Christ—our only hope of pardon from the punishment we deserve for breaking the law. This is because He perfectly kept the law in our place and received the punishment for our sin.
Secondly, the law acts to restrain evil. John Calvin called this its "civil use." The law by itself has no power to change the heart, but it restrains evil by its threat of judgment (Deut. 19:16-21; Rom. 13:3, 4) For example, secular governments recognize the need to have moral laws to govern our society. They have punishments for stealing, murder, etc. They even recognize the importance of telling the truth—there is punishment for those who testify in the court of law and are found to have lied to the court. In this way the law serves to protect the righteous from the unjust.
Thirdly, the law acts to guide the Christian into the good works that God has planned for them: “For we are his workmanship, created in Christ Jesus for good works, which God prepared beforehand, that we should walk in them” (Eph. 2:10). So the law reveals to the Christian what pleases their heavenly Father, and as the Psalmist, they cry out, "Oh how I love your law! It is my meditation all the day" (Ps. 119:97). The Christian does not obtain salvation through the keeping of the law, (Rom. 7:4, 6; Gal. 3:25) rather his obedience to God's law is the proof of his love for Him. "If you love me, you will keep my commandments” (John 14:15). As we love God and keep His commands, we bring Him honor and glory.
Question fifteen of The New City Catechism addresses this very subject of God's law, it reads as follows:
Question: Since no one can keep the law, what is its purpose?
Answer: That we may know the holy nature and will of God, and the sinful nature and disobedience of our hearts; and thus our need of a Savior. The law also teaches and exhorts us to live a life worthy of our Savior.
On the one hand, by studying God's law, we learn what offends God, and on the other hand, we see what pleases Him. God's moral law is for our own benefit and it is eternal, it is always binding upon us. “Blessed are those whose way is blameless, who walk in the law of the Lord! Blessed are those who keep his testimonies, who seek him with their whole heart” (Ps. 119:1). As sons of God, we have been redeemed from the curse of the law, but not from our duty to love and obey it. We have been justified, not because we obeyed God's law, but in order that we might become obedient to it out of a heart that loves God and delights in it.
Only when we love God will we obey His commands and be able to truly proclaim "Let your mercy come to me, that I may live; for your law is my delight" (Ps. 119:77).