by J. I. Packer
I would not have known what sin was except through the law. ROMANS 7:7
Scripture shows that God intends his law to function in three ways, which Calvin crystallized in classic form for the church’s benefit as the law’s threefold use.
Its first function is to be a mirror reflecting to us both the perfect righteousness of God and our own sinfulness and shortcomings. Thus “the law bids us, as we try to fulfill its requirements, and become wearied in our weakness under it, to know how to ask the help of grace” (Augustine). The law is meant to give knowledge of sin (Rom. 3:20; 4:15; 5:13; 7:7-11) and, by showing us our need of pardon and our danger of damnation, to lead us in repentance and faith to Christ (Gal. 3:19-24).
Its second function is to restrain evil. Though it cannot change the heart, the law can to some extent inhibit lawlessness by its threats of judgment, especially when backed by a civil code that administers present punishment for proven offenses (Deut. 13:6-11; 19:16-21; Rom. 13:3-4). Thus it secures some civil order and goes some way to protect the righteous from the unjust.
Its third function is to guide the regenerate into the good works that God has planned for them (Eph. 2:10). The law tells God’s children what will please their heavenly Father. It could be called their family code. Christ was speaking of this third use of the law when he said that those who become his disciples must be taught to keep the law and to do all that he had commanded (Matt. 5:18-20, 28:20), and that it is obedience to his commands that will prove the reality of one’s love for him (John 14:15). The Christian is free from the law as a supposed system of salvation (Rom. 6:14; 7:4, 6; 1 Cor. 9:20; Gal. 2:15-19; 3:25) but is “under Christ’s law” as a rule of life (1 Cor. 9:21; Gal. 6:2).
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From: Concise Theology: A Guide To Historic Christian Beliefs