When in the Lutheran Church…the one-sided antinomian movement, which wanted to have the preaching of the law banned from the church as not at all applying to Christians and being harmful to true faith, was brought back onto the right track by Luther, the doctrinal designation of a threefold use of the law, as the Formula of Concord has it, appeared: usus politicus [the curb function], usus elenchticus or paedeuticus [the mirror function] and usus normativus [the guide function]. The last, also called usus tertius, concerns believing regenerated Christians. The law contains the norm according to which believing Christians are to regulate their lives.
In general, the Reformed teach the same. They too had to combat various types of antinomianism which developed in their midst, even though it was derived in part from other sources. When they talk about the enduring authority of the law, however, they mean something different from what Lutherans mean. If we take a closer look at how Lutherans think of this enduring authority, then it cannot consist in this that the law maintains its compelling and demanding authority over believers. For just as Christ redeemed us from the curse, so he also redeemed us from the compelling demands of the law in that he did everything which the law demands of us. Consequently, the believer as such consciously knows that by virtue of his justification he has been liberated from both aspects of the law, from its curse (maledictio) and its coercion (coactio).
It is therefore completely in keeping with the Lutheran point of view when [F.A.] Philippi in his book about the active obedience of Christ says that the one who has been justified is always conscious of freedom from both the punishment and the demands of the law, from the curse and the coercion of the law.This is the same as saying that both forgiveness of sins and justification have been equally granted to him. The believer, as a person who has been justified, is no longer under the law. The law does not have to urge him on to anything anymore; it has nothing to demand of him anymore because its demands have been fulfilled by Christ. What motivates the Christian is love, which has its origin in faith and is the fruition of that faith, in short, the Holy Spirit. Faith, which by the grace of God is saturated with love, compels the believer of himself to do that which is good.
If, however, the law is still to be a norm for him to which he knows he is subject, to the observance of which he knows he is obligated, to whose command he knows he is bound, then this can happen only in so far as he has yet another side than that according to which he possesses the conviction of his justification, and in faith through the Holy Spirit also possesses the immanent principle of conduct well-pleasing to God. That is, the law applies as law only to the Old Man, who is still always present, to the sinful part of the believer, which has not yet been overcome and assimilated by the Spirit.
Gerhard expresses this as follows:
“The regenerate Christian, in so far as he is a Christian and regenerated, needs no law, namely, that drives and compels him, because he does good works on his own initiative. Since, however, he is not yet completely regenerated but is still partly under the old domination of the flesh, his stubborn flesh must be compelled by commands and threats and subjected to the rule of the spirit.”
Luther had already made essentially this same point against Agricola. So this teritus usus legis or normativus is very closely related to the paideuticus in that it has reference only to sin, to its subjection and elimination. Only sin, which is still continually present, makes such a positive norm necessary.
Reformed doctrine too is acquainted with this purposely negative use of the law. It likewise had reason to remind the antinomians that the believer and regenerate Christian is not yet perfect or free from everything sinful. But this significance of the law and the believer’s need for it is still not the complete statement of Reformed doctrine. It is merely the subordinate aspect of it. The law has a positive significance for the regenerate person as such, not just a negative one, in so far as he still has a side that has not yet been renewed. It must tell him what he as a believer and regenerate person must do. It must prescribe God’s will to him and encourage him to carry it out. Accordingly, the justified and regenerated person as such needs the law, and for this reason that, as we have seen before, he has to do good works and in doing the same has to work out his salvation.
The law is the rule of good works, as [Benedict] Pictet teaches:
[To be sure, the law] no longer has that use which it would have had in the state of innocence, where it would have been the means to obtain eternal life [this, by the way, is also a specifically Reformed idea of which the Scriptures say not a word— August Pieper], also with reference to justification. For believers are no longer under the curse of the law, but the law is not abolished; it is always the most perfect rule for morals. Christ and the apostles recommend the law, and without sanctification no one will see the face of God.
The Geneva Catechism states: “Why then are there so many admonitions, commands and exhortations, which both the prophets and apostles employ everywhere? They are nothing else than expositions of the law, which lead us to obey the law rather than away from it.” Lutherans speak here not of law (lex) but of [evangelical] admonition (mandatum), etc. Pictet says, “Christ has redeemed us from the yoke of the curse of the law but not from the necessity of rendering obedience to God.” It belongs to the kingly office of Christ to urge his people according to God’s eternal law. An abrogation of the moral law is not possible because “it is based for the most part on the natural justice of God. Other laws are based on the positive justice of God, which depends on the mere will of God.” [Anton] Hulsius too teaches similarly: “Nevertheless, while the use of the law does not cease, namely, that through it prior to faith the spirit of servitude might produce the beginnings of conversion, just as after faith, when the spirit of servitude has already been changed into the spirit of adoption, the same law is a mirror of gratitude for the liberation through Christ.” And [Peter van] Mastricht says, “The chief norm of obedience is the divine law” as “a prescription of duty under the threat of punishment for the scorner.” The obedience is “that part of spiritual life by which the Christian is inclined to carry out the will of God, with subjection through faith, to the glory of God.” Obedience has its origin “in the regenerating, converting and sanctifying grace of God, as well as in faith.”
It could not be expressed more definitely than this, namely, that the law applies to the believer as such, while Lutherans declare that the believer is free from the coercion (coactio) and threat (comminatio) of the law in that he as a believer voluntarily (sua sponte) does what is Godpleasing. Consequently, he does not need the external prod of a demanding law standing over him. Only because the believer as he is in this life (in concreto) is also something else besides a believer does the law still also apply to him to convict him of sin. The Reformed, on the other hand, let law apply to the believer because and in so far as he is a believer. For God wants nothing from us except that we follow the law, as the Geneva Catechism states; and because we can never follow it completely, it keeps pride down by means of its constant condemnations. “Finally, the law serves them as a curb by which they are kept in the fear of God.”
– Matthias (Max) Schneckenburger, “Vergleichende Darstellung des lutherischen und reformierten Lehrbegriffs” (A Comparative Presentation of the Lutheran and Reformed Concept of Doctrine), quoted in “The Difference Between The Reformed And The Lutheran Interpretation Of The So-Called Third Use Of The Law” by August Pieper