This threefold use or functions of the law is crucial. First, here’s Berkhof:
1. A usus politicus or civilis.
The law serves the purpose of restraining sin and promoting righteousness. Considered from this point of view, the law presupposes sin and is necessary onaccount of sin. It serves the purpose of God’s common grace in the world at large. This means that from this point of view it cannot be regarded a means ofgrace in the technical sense of the word.
2. A usus elenchticus or pedagogicus.
In this capacity the law serves the purpose of bringing man under conviction of sin, and of making him conscious of his inability to meet the demands of thelaw. In that way the law becomes his tutor to lead him unto Christ, and thus becomes subservient to God’s gracious purpose of redemption.
3. A usus didacticus or normativus.
This is the so-called tertius usus legis, the third use of the law. The law is a rule of life for believers, reminding them of their duties and leading them in the way of life and salvation. This third use of the law is denied by the Antinomians.
Berkhof’s Systematic Theology, 614-615
J.V. Fesko also summarizes the three functions of the law below:
Based upon such passages of Scripture [Gal. 3:18-26, Romans 7:12, James 2:8], the Reformed church teaches there are three functions of the law:
(1) political -to restrain evil in the public realm;
(2) pedagogical -the guardian aspect of which Paul speaks, which drives us to Christ;
and (3) normative – the Law no longer condemns the believer because of the work of Christ but is now a guide for Christian behavior
(cf. Westminster Confession of Faith 19.6) (16).
Taken from J.V. Fesko’s The Rule of Love: Broken, Fulfilled, and Applied
Notice also below with J. Gresham Machen in how he makes the important distinction between the pedagogical (second) and normative (third) uses of the law:
“Unlike Jesus, we are sinners, and hence, unlike Him, we become Christians; we are sinners, and hence we accept with thankfulness the redeeming love of the Lord Jesus Christ, who had pity on us and made us right with God, through no meritof our own, by his atoning death.
That certainly does not mean that the example of Jesus is not important to the Christian; on the contrary, it is the daily guide of his life, without which he would be like a ship without a rudder on an uncharted sea. But the example of Jesus is useful to the Christian not prior to redemption, but subsequent to it.
In one sense indeed it is useful prior to redemption: it is useful in order to bring a sinful man into despair of ever pleasing God by his own efforts; for if the life of Jesus be the life that God requires, who can stand in His holy presence? Thus to the unredeemed the example of Jesus has an important part inthe proclamation of that terrible law of God which is the school master to bring men unto Christ; it serves by its lofty purity to produce the consciousness ofsin and thus to lead men to the Cross.” (111)
Taken from Machen’s What is Faith?
Also worth noting, here’s Mike Horton quoting Calvin on the second use of the law.
“Calvin said, ‘Whenever you need to know what you need to do—go to the law, but whenever you hear the law threatening you, you must set the law aside and tell the law I will not hear your thunderings and flee to Christ and listen to him only.'”
Taken from the broadcast Rightly Dividing the Word: Law & Gospel. (1)
Horton, in quoting Calvin, on the broadcast was talking about the normal Christian life found in Romans chapters 6-8. As Kim Riddlebarger mentions(2) that Romans 7 is speaking about the intense struggling and battling that the Christian has with sin. And how the Justified-sinner again flees to Christ:
“Wretched man that I am! Who will deliver me from this body of death? Thanks be to God through Jesus Christ our Lord! So then, I myself serve the law of God with mymind, but with my flesh I serve the law of sin. There is therefore now no condemnation for those who are in Christ Jesus.” (Rom. 7:24-8:1)
Turning again, to Fesko to elaborate on this point. Is the 2nd use of the law—the pedagogical use also for the Christian? Yes.
The third use of the law of God, a guide, is for the Christian (the Justified-sinner) only. The second use of the law that drives us to Christ is for both the unbeliever and the regenerate believer.
“In examining the first commandment, we see the second use of the law, namely, the command to worship God only and to have no other gods before Him. When the Holy Spirit applies this commandment to the heart of the unbeliever in regeneration or even the regenerate believer, He causes the person to flee to Christ. Moreover, the Spirit enables us to love the triune God and in this way applies the rule of love to our hearts. We hear the demands of the Law, recognize our sin in worshipping other gods, and flee to Christ, the revelation of the one true God. We then recognize the third use of the Law, namely that the Law is a guide to us for holy living (see Westminster Confession of Faith 19.6).” (27)
Taken from J.V. Fesko’s The Rule of Love:Broken, Fulfilled, and Applied
“The poor, struggling sinner who is erroneously told that the struggle with sin he or she is currently experiencing is a signof defeat and that the person is not yet a Christian, or else has chosen not totake advantage of the victory offered to all those in Christ, should instead see the struggle with sin as proof that sanctification is actually takingplace.”