The Law of God has a few purposes in the life of the believer. Here in the fifteenth question of the New City Catechism we find two of those purposes spelled out. First, we see the moral use of the law. God’s law spelled out in Scripture and written on our hearts restrains us from diving too deeply into sin. An example of this is the law requiring a certificate of divorce. When asked about this law, Jesus replies “Because of your hardness of heart he wrote you this commandment (Mark 10:4).” This law was meant to restrain the evil of divorce. And in its restraining nature, the law also reveals our need for a Savior. When we realize how much we have rejected God and His law, it leads us to the realization that we can never overcome our disobedience, and therefore must be saved by something outside of us rather than something inside of us. Second, we see the pedagogical use of the law or the teaching function of the law. The law of God teaches us about God and His nature. God’s laws are not some arbitrary list of rules, but an outline of how God created us to live and how to live in relationship with Him. The law teaches us how to truly love God and to relate with God. If it were not for the law we might love God in any way we choose thereby elevating our preferences above God. Therefore the Law of God teaches us about God and offers guidance into loving Him and living with others.
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.