Q 15: 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.
John Calvin’s magisterial, two-volume Institutes of the Christian Religion begins with the following words: “Nearly all the wisdom we possess, that is to say, true and sound wisdom, consists in two parts: the knowledge of God and of ourselves.” And though we can arrive at some partial knowledge of both God and ourselves through our own reflection and observation (this is what theologians refer to as "general revelation"), we nonetheless require instruction in what we cannot ourselves apprehend (this is what theologians call "special revelation"). God’s law supplies this instruction. It first reveals to us Who God is by showing us what He expects of us, and then exposes our own inability to meet His standard. The law teaches us that God is righteous and we are sinners.
But of course, this is not the end of the matter. In teaching us that we cannot meet God’s standard for righteousness, the law also points to our need for a Savior, for the One Who did meet the Father’s standard and Who paid the penalty for our failure to do so. As the Apostle Paul writes in his letter to the Galatians, “the law was our tutor to bring us to Christ, that we might be justified by faith.” Salvation comes not when we perfectly obey God’s law—for of course we cannot do so; rather, salvation comes when we accept, in faith, the glorious, finished work of Christ, who accomplished for us what we cannot accomplish for ourselves.