Great question! To be honest, I am not 100% sure about the whole debate myself. I have found it rather difficult to keep up with the plethora of terms and their definitions that have been floating around on the internet (i.e., Gospel Reductionist, Law-Gospel Reductionist, Radical Lutheran, Soft Antinomian, etc…) Unfortunately, debates and conversation on the internet and social media can be difficult; conversations can also get lost in translation. However, with that said, I can speak to your question on the third use of the law.
My friend Dr. Jack Kilcrease has a paper that he wrote in critique of Gerhard Forde’s doctrine of the law. Within his paper there is an excellent portion where he expounds on the proper understanding of the third use of the law according to the Formula of Concord. I have found it most helpful and I believe you will too. It is printed below.
Grace and peace,
Zion Lutheran Church of Gwinner, ND
Excerpt take from:
Jack Kilcrease, “Gerhard’s Forde’s Doctrine of the Law: A Confessional Lutheran Critique,” Concordia Theological Quarterly Volume 75:1-2 (2011): 171-174.
…The Formula of Concord, following Luther, teaches that the law is God’s eternal and objective will, which is revealed to his creatures through both nature and the Scriptures. Because of the fall, this revelation becomes restraining and condemning. Nevertheless, it is no less a revelation of God’s will. According to the sixth article of the Formula of Concord, the law possesses a first use: “external discipline and decency are maintained by it against wild, disobedient men.”(FC SO VI; Triglot 963) The Formula here specifically defines the first use as applying to non-Christians, or at least to false ones, through coercive authorities (parents, teachers, police, military, etc.). It is not meant to instruct or discipline Christians, but non-believers who are “wild and disobedient.”
The sixth article of the Formula also defines the third use of the law. This logically follows from the contention of both Scripture and the Lutheran Confessions that the law is God’s eternal will for human beings. Although human beings are no longer defined and determined in their relationship with God (coram deo) by the condemnation of the law, the law nevertheless still represents God’s will for human life: “For the law is a mirror in which the will of God, and what pleases Him, are exactly portrayed, and which should [therefore] be constantly held up to the believers and be diligently urged upon them without ceasing.”(FC SO VI; Triglot 963) Such a formulation provokes the question: if faith sanctifies and renews Christians, will they not automatically perform the works of the law? Yes, to the extent that they are sanctified, they will perform the works of the law, but “believers are not renewed in this life perfectly or completely.”(FC SO VI; Triglot 965) The justified sinner, therefore, is in need of the law to subdue his or her old nature. The Formula of Concord compares the old nature to “an intractable, refractory ass [that] is still a part of them [believers], which must be coerced to the obedience of Christ, not only by the teaching, admonition, force and threatening of the Law, but also oftentimes by the club of punishments and troubles, until the body of sin is entirely put off.”(FC SO VI; Triglot 969) This use of the law is no more harmless than any other use of the law. It cannot rightly be characterized as a pleasant or non-threatening form of the law.
The second point made by the Formula is that justified sinners renewed by sanctification need the law as instruction so that they do not engage in “self-chosen worship, without God’s Word and command.”(FC SD VI; Triglot 969) In other words, although the regenerate person desires to do good works, he does not automatically know which works are God-pleasing. This strikes a similar note to that of the Augustana with characterization of late medieval Roman Catholicism as encouraging “childish” and “needless”(AC XX; Triglot 53) works such as pilgrimages, praying the rosary, etc. God desires specific works, and engaging in works of devotion not commanded by God is useless. Because of this, the law cannot simply be understood as existing relative to sin. Rather, it is also a necessary part of living in creation this side of the eschaton. For human beings to rule in creation as God intends, they must have specific regulations to direct their business as the caretakers of the created order. The only thing that could abrogate this would be the passing away of the old creation at the eschaton. This fits very well with Luther’s remarks in both the Genesis lectures and the Antinomian Disputations.
Therefore, when the Formula of Concord posits a third use of the law, it is not supplementing a weak connection between justification and sanctification by trying to inculcate obedience to the law. Neither does it attempt to claim that the law has suddenly become friendly and nonthreatening. The confessors of the Formula thoroughly agree with the young Melanchthon’s “lex semper accusat.” In reality, the Formula has attempted to take seriously the simul of Christian existence. On the one hand, the Christian lives in the old creation regulated by the law and the orders of creation and, therefore, needs practical instruction in God’s will. On the other hand, Christians have already received Christ’s alien righteousness and been sanctified by faith. They have been proleptically translated into heaven with its lex vacua. To describe this situation in Paul’s terms, the Christian is sanctified and can say “in my inner being . . . I delight in God’s law” (Rom 7:22). At the same time, the Christian does what he hates (Rom 7:23). For this reason, Christians must” discipline the flesh” (1 Cor 1:27).