Natural Law and Moral Law: How the Decalogue Retains Its Moral Relevance

What relevance do the Ten Commandments have for Christians and society today? To answer that question, we must go upstream and consider how God reveals his moral order in both the world and the Word.¹ The Decalogue did not create a new morality at Sinai. Rather, it particularized in a covenant the natural law already revealed and established in creation and grounded ultimately in God’s own holy character.² Understanding this relationship between creation and covenant helps explain why the law given through Moses retains moral relevance even after the Mosaic covenant has reached its fulfillment in Christ. Because natural law precedes Sinai, it endures beyond the abrogation of the Mosaic covenant.

Understanding natural law as part of God’s enduring creation order helps situate the Decalogue for modern use while answering Protestant concerns that a robust appeal to law risks smuggling works back into the doctrine of justification. Modern Reformed evangelicalism often hesitates to adopt natural law theology. Many Protestants hear the language of natural law and instinctively associate it with Roman Catholic soteriology, fearing that it threatens the Reformation’s insistence on justification by faith alone.³ But that concern diminishes when natural law—and Mosaic law—is situated within the Bible’s unfolding covenantal structure and read redemptive-historically.⁴ Accordingly, this essay argues that Christians ought to receive the Ten Commandments as a covenantal expression of the natural law, no longer binding as the Mosaic law-covenant, but fulfilled in Christ and enduring as authoritative Scripture that bears witness to God’s holy character.⁵

I will make this case in three movements. First, I will situate the Decalogue within the magisterial Reformers’ retrieval of natural law, showing how they treated the Ten Commandments as an enduring summary of the moral law for the Christian life. Second, I will consider how Paul applies the Mosaic law in the new covenant, arguing that although Sinai has been abrogated as law-covenant, it retains enduring moral relevance as Scripture. Third, I will argue that this moral order is finally understood in and through the person and work of Jesus Christ, who fulfills the law through his active and passive obedience and directs its moral demands toward their eschatological consummation.

Magisterial Reformers

Modern Protestant unease with natural law and the continuing moral use of the Mosaic law should not be mistaken for the posture of historic Protestantism toward either. Quite the opposite: the Reformation inheritance is thick with esteem for the Ten Commandments as a summary of God’s moral law, and even a brief survey of the magisterial Reformers proves this. This section will briefly survey how Martin Luther, John Calvin, and Franciscus Junius understood the relationship between the moral law and natural law.

Martin Luther begins this historical survey by affirming the continuing relevance of the Mosaic writings for the constituents of the new covenant community, but with clear qualifications. His adamant insistence that Gentile Christians are not, and never have been, under the Law of Moses did not prevent him from affirming its relevance for Christians under certain conditions. For Luther, that relevance depended upon the enduring natural law. In his sermon “How Christians Should Regard Moses,” Luther wrote that Christians may regard Moses as a teacher, but not as a lawgiver, “unless he agrees with both the New Testament and the natural law.”⁶ Luther’s other comments on the Decalogue draw the same jurisdictional line. “The Law of the Decalogue has no right to accuse and terrify the conscience in which Christ reigns through grace, for Christ has made this right obsolete.”⁷

Luther then narrows his focus to the moral law inscripturated in the Ten Commandments, distinguishing it from the commands given particularly to ancient Israel:

Therefore it is natural to honor God, not steal, not commit adultery, not bear false witness, not murder; and what Moses commands is nothing new. For what God has given the Jews from heaven, he has also written in the hearts of all men. Thus I keep the commandments which Moses has given, not because Moses gave commandment, but because they have been implanted in me by nature, and Moses agrees exactly with nature . . . But the other commandments of Moses, which are not [implanted in all men] by nature, the Gentiles do not hold . . . Now this is the first thing that I ought to see in Moses, namely, the commandments to which I am not bound except insofar as they are [implanted in everyone] by nature [and written in everyone’s heart].⁸

For Luther, then, the continuing moral relevance of the Decalogue is indexed to natural law: what binds the Christian is not Mosaic jurisdiction as such, but the moral order written by God on the human heart.

John Calvin also carries forward this deep appreciation for the relationship between natural law and the moral law. He correlates the natural law written on the heart with the moral law expressed in the Ten Commandments.⁹ As Calvin explains, “the very things contained in the two tables are, in a manner, dictated to us by that internal law” written and stamped on every heart.¹⁰ The Decalogue, then, did not create a new morality at Sinai but gave covenantal expression to a moral order already woven into creation and revealed through it. Calvin makes the relationship explicit elsewhere: “the law of God which we call moral, is nothing else than the testimony of natural law.”¹¹ He then points to the laws of other nations against murder, adultery, and theft as evidence that this “divine law” is accessible beyond Israel.¹² For Calvin, the Decalogue therefore bears witness to the moral order God has woven into creation itself.

A final observation comes from one of Calvin’s students, Franciscus Junius, who averred that the law of Moses provided a perfect example of natural law.¹³

The law of Moses is an example of the natural law, most suitably expressing the common notions of nature endowed by the natural law. For the principles and conclusions of the natural law, that is, those common notions of nature, are perfectly expressed in the law of Moses. These common notions not only exist in the corrupted nature of man, but also existed in the pure and original nature.¹⁴

Taken together, these magisterial Reformers demonstrate that Protestant regard for the Ten Commandments as an expression of God’s moral law is no modern innovation. The Reformers understood the moral law expressed in the Decalogue to reflect a natural law that precedes the Mosaic covenant and therefore persists beyond it. They discerned the same moral order at work in the world and written in the Word. Following their line of thought, I will now consider Scripture’s own account of the Decalogue’s continuing moral relevance.

Paul and the Law

“Now we know that the law is good, if one uses it lawfully . . .” (1 Tim. 1:8). This section argues that Christians must interpret the Decalogue according to its covenantal location, in light of the natural law that precedes it, and in light of its interpretation and application in the New Testament. Although the coming of Christ has brought Sinai’s law-covenant to an end, the Mosaic law remains authoritative Scripture and therefore retains enduring moral relevance for the new covenant community. In other words, much of what can be said about Sinai’s law can also be said analogically of the natural law. Neither can justify or save, yet both remain good when used lawfully: they bear witness to God’s moral order, expose human rebellion, and direct human life toward creaturely flourishing.

The argument warrants one clarification here. This essay uses “natural law” to refer to natural law qua creation’s moral order, not to a category of moral law within the Mosaic covenant.¹⁵ Progressive Covenantalism, which this essay assumes, treats the Mosaic law as a unified whole that reaches its telos in Christ, not as three legal strands carried forward or discontinued piecemeal.¹⁶

Brian Rosner’s Paul and the Law provides an important framework for understanding the enduring moral relevance of the Decalogue. Rosner writes to answer the question: “What does the Jewish apostle to the Gentiles say about the Law of Moses in relation to believers in Christ?”¹⁷ His answer directly aids this essay’s argument concerning not only the relationship between Mosaic law and natural law, but also the enduring relevance of both for Christians today. In sum, Rosner argues that “there is a sense in which the Law as Mosaic covenant is abolished, but the Law as Scripture has ongoing value for Christians.”¹⁸

Rosner holds together the continuing authority of the Mosaic law as Scripture with its covenantal limitation: Gentiles were never under Sinai, and Christ has brought the Mosaic law-covenant to an end. He reads Leviticus 18:5 as summarizing its covenantal principle, “do this and live,” a principle Paul no longer applies as the governing covenantal arrangement for God’s people.

Rosner’s argument turns on Paul’s treatment of the Mosaic law as a covenantal whole. Paul does not divide the law into individual declensions of moral, ceremonial, and civil law. Rather, he interprets the whole in light of its fulfillment in Christ. This does not mean, however, that the Mosaic law is no longer authoritative Scripture.¹⁹ Paul reads the law in light of its fulfillment in Christ and continues to use it as wisdom and prophecy for God’s people. Its authority for believers, therefore, is understood through Christ and the new covenant, not by separating out one part of the Mosaic law that remains in force.

Constituents of the new covenant community are not under the law as law-covenant. Instead, they fulfill its righteous intent in their new life in Christ, as God writes his law on their hearts, which in turn leads them to lives of Spirit-empowered obedience.²⁰ Said differently, they do not obey to be saved; they obey because they have been saved. “With Paul’s teaching on the Christian fulfilment of the law we meet the basis of our justification, in Christ’s perfect obedience on our behalf, our safe position of being in union with him, and the fact that our new status leads to a new way of living, in love as the fulfilment of the law.”²¹ This brings the argument to its final movement: the enduring moral order disclosed in the Decalogue finds its fulfillment and Christ-exalting terminus in the person and work of Jesus Christ, whose Spirit enables the new covenant people to fulfill the law’s righteous intent.

Christotelic Fulfillment

This Christotelic understanding of the law also finds deep roots in the Christian tradition.²² Aquinas, for example, locates the law’s fulfillment in Christ’s own active obedience:

And Christ, indeed, wished to conform His conduct to the Law, first, to show His approval of the Old Law. Secondly, that by obeying the Law He might perfect it and bring it to an end in His own self, so as to show that it was ordained to Him. Thirdly, to deprive the Jews of an excuse for slandering Him. Fourthly, in order to deliver men from subjection to the Law, according to Gal. 4:4–5.²³

Taken together, Christ’s active obedience in fulfilling all the demands of God’s law and his passive obedience in bearing its penalty are indispensable for understanding the continuity of God’s moral order. Grounded eternally in God’s character, that order precedes Sinai and endures beyond the Mosaic covenant. Christ does not abolish righteousness. He embodies it perfectly, bears the judgment due to sinners, and brings the Mosaic law-covenant to its appointed end. Through his life, death, and resurrection he accomplishes the work that will reach its final consummation at his return.

Paul’s use of the Decalogue in Romans 13 provides a key test case for this essay’s argument concerning the enduring relevance of God’s moral order for Christians and society today. Love fulfills the law not by bypassing moral order, but by bringing the regenerate person into rightly ordered participation in it: “Owe no one anything, except to love each other, for the one who loves another has fulfilled the law” (Rom. 13:8).²⁴ After listing several Decalogue commands, Paul concludes, “Love does no wrong to a neighbor; therefore love is the fulfilling of the law” (Rom. 13:10).

Earlier in Romans, Paul has already established that Jew and Gentile alike stand condemned before God, whether judged by the Mosaic law or by the law written on the heart (Rom. 2:12–16; 3:9–20). However, it is toward such sinners that God supremely displays his love: “but God shows his love for us in that while we were still sinners, Christ died for us” (Rom. 5:8). Jesus alone perfectly fulfilled the righteousness disclosed in both the natural law and the Mosaic law. In his life, he fulfilled their moral demands; in his death, he bore the judgment due to those who had violated them. Thus, both the Mosaic law and the natural law disclose God’s moral ordering of human life in and through his Son, directing us toward love of God and neighbor. The Decalogue’s two tables reflect this same order.

So, too, natural law remains relevant in the present age, though its operation must be understood according to covenantal location. Within the new covenant community, natural law is not discarded but illuminated by Scripture, fulfilled in Christ, and received under the law of Christ. Both inside and outside the new covenant community, natural law remains the moral grammar, originating in creation, by which civic life may be ordered toward creaturely flourishing. Abiding by natural law will not save, but it will ordinarily conduce to temporal goods. One who does not murder, steal, lie, or covet will, of course, live better than one who does. A society that broadly conforms its laws to natural law will fare better than one that devotes itself to codified vice. The natural law theologian, then, does not want less than what natural law offers. He wants more: not merely ordered creatures, but redeemed creatures, who fulfill the intent of the law through their Christ-honoring obedience.

No degree of conformity to God’s moral pattern, ingrained on the conscience and patterned in creation, can bring about salvation. Temporal flourishing cannot justify the sinner or reconcile him to God. To embrace the moral form of natural law while rejecting its telos in Christ is to live anachronistically, as though law could secure the life to which it points. It cannot. In this analogical sense, natural law, like the Sinai covenant, points beyond itself. Although Paul’s guardian language in Galatians refers directly to the Mosaic law, natural law performs an analogous function: it exposes creaturely accountability, discloses moral order, and leaves humanity restless apart from Christ, who fulfills both the Mosaic law and the natural law.²⁵

Conclusion

The Ten Commandments retain material relevance for Christians and society alike. Although they no longer govern God’s people as the covenant stipulations of Sinai, they give authoritative expression to a moral order grounded in God’s own character of love and holiness and embedded in creation itself. For this reason, the Decalogue continues to instruct the church and to bear public witness to the moral order by which God holds all people accountable. However, the Decalogue also points beyond itself to the person and work of Jesus Christ. In him, the law’s demand for righteousness and its curse upon disobedience find their fulfillment: Christ perfectly obeys where sinners fail to conform to the moral order disclosed in both the Decalogue and natural law, and bears the judgment they deserve, securing the flourishing of his people and redounding finally to the glory of God.