The first clause of this question effectively summarizes Questions 13 and 14. Because Adam’s original sin as the father and representative of all mankind, we are each born in sin and corruption, who no longer possess the ability to keep God’s law. You will notice, however, that the Bible repeatedly praises and rejoices in God’s law and summons God’s people to both love it and keep it. Chief examples of this are Psalms 1, 19, and 119. But what good is God’s law to us if we cannot keep it? And how are we supposed to love God’s law if it only testifies to our sinfulness and rightful condemnation?

In the catechism’s answer, we are given three purposes that the law of God serves. First, that we may know the holy nature and will of God. This means that the law has a purpose in teaching us about God. Particularly, it reveals two aspects of God: His nature and His will.

The law reveals God’s nature because His law is a reflection of Himself as the Lawgiver. This is why when preaching through the Ten Commandments I aimed to show how each displayed an attribute of God. For example, the First Commandment’s decree of exclusive worship reflects God’s holiness, that there is none like Him. Likewise, the Second Commandment reflects that God is spirit and must be worshiped in spirit and in truth. God’s faithfulness, graciousness, truthfulness, and sufficiency are reflected by the Seventh, Eighth, Ninth, and Tenth Commandments.

But the law also reveals God’s will. The very nature of the commandments testifies to this. In commanding us not worship anyone or anything else, God is making His will known to us. We do not need to guess or wander into mysticism to know God’s will. God’s law is Him telling us quite plainly what pleases Him and what grieves and angers Him.

The second purpose of God’s law is that we may know the sinful nature and disobedience of our hearts; and thus our need of a Savior. This ought to be the most obvious use of God’s law, for even a glance the law should reveal that we fall woefully short of God’s holy will. Indeed, Paul even goes so far as to say in Romans 3:20 that “through the law comes knowledge of sin.” It is only by God’s law, written in Scripture but also written imperfectly upon each human’s conscience (see Romans 2:15-16), that we are ever able to recognize our sin for what it is. Even with the guilt of our conscience, we very often justify our own sins; thus, we should not be surprised that without the law we would be so steeped in sin that we would have no knowledge that it was sin.

Yet because the guilt of our conscience cries out against us whenever we transgress against God’s law, our need of a Savior is also revealed. Any honest assessment of ourselves against God’s holy standard will find us coming up wholly and completely short. We cannot keep only the First Commandment for an entire day, let alone perfectly since we were born. No, gazing into God’s law reveals that we are entirely unable to save ourselves. Either God must make a way to redeem us, or we will be justly damned by Him.

The law’s third purpose reminds us that we do indeed have such a Savior and that the law also teaches and exhorts us to live a life worthy of Him. In both the Westminster and Heidelberg Catechisms, the Ten Commandments are presented towards the end as a portrait of how the life of a Christian ought to be lived in obedience to God and in thankfulness to Christ Jesus our Lord. I would imagine that it has been placed here towards the beginning of the New City Catechism because the Ten Commandments are not as commonly known today as they once were. Thus, most people today need a presentation of them to know exactly why they need a Savior in the first place. I agree with that decision; however, we should diligently note that the law is not merely to display our need of a Savior but to also show us how to live a life worthy of that same Savior.

This purpose seems to be often ignored today as many Christians wrongfully label any diligent pursuit of holiness to be legalism. Yet the Scripture is clear that Christians ought to strive to obey God’s law. Such striving becomes certainly becomes legalism if we attempt to earn God’s favor through it, but the striving itself is not the issue. As Ephesians 2:8-9 makes clear, we are saved by God’s grace, not by any works of our own. However, in verse 10, Paul does tell us that we were “created in Christ Jesus for good works, which God prepared beforehand, that we should walk in them.” Only Christ’s finished work upon the cross saves us, yet He has saved us for the purpose of now walking in obedience to Him.

Indeed, rather than the gospel being an abolition of the law, as many Christians seem to think, the gospel eliminates our debt for breaking God’s law and writes His law upon our hearts, giving to us the freedom to actually choose obedience. Since we no longer have any fear of the condemnation that the law brings, we are free to see the beauty of the law as it reveals the will of our Father. Thus, it is in the law that we discover how to “be imitators of God, as beloved children” (Ephesians 5:1). Indeed, the cry of all followers of Christ ought to be: “Jesus paid it all. All to Him I owe!”