“Natural law cannot contradict the Bible. So why depend on a vague concept of natural law when we can trust in the perspicuous revelation of God’s Law?“
First, what is Natural Law?
Natural Law is the reflection of God’s moral character and the moral order of creation, as designed by God, which is written on the human heart and evident through the light of nature (Rom. 2:14-15; Rom. 1:19; 1 Cor. 5:1), but held in unrighteousness (Rom. 1:18; Jer. 17:9; Prov. 14:12), whose substance is no different than the ten commandments. It is “the practical rule of moral duties to which men are bound by nature” (Turretin, Institutes, XI.i.5). It is not called “natural” because it originates from nature apart from God, but because it is revealed by God through nature; and that in two ways: 1) it is innately known in the conscience and 2) it is acquired by sound reason about the created order. If one believes in Natural Revelation then one must necessarily believe in Natural Law, because law is the moral imperative that necessarily follows that revelation.
Natural Law is materially the same as the moral law summarized by the ten commandments. God has given us both modes of revelation, natural and special, to know his law. Therefore it is a false dichotomy to set Natural Law and Scriptural Law in opposition to one another. Taking Scripture seriously does not require rejecting the existence, nor downplaying the importance, of Natural Law. Both need to be understood in their proper place, including how each apply to civil government. In affirming Natural Law, we are not saying that Scripture is irrelevant, far from it, we are simply saying that Natural Law and Scripture were intended to complement one another.
Second, what Natural Law is not.
1) Natural Law is not the Gospel and was not intended to save.
“They who, having never heard the gospel, know not Jesus Christ, and believe not in him, cannot be saved, be they never so diligent to frame their lives according to the light of nature, or the laws of that religion which they profess; neither is there salvation in any other, but in Christ alone, who is the Saviour only of his body the Church (Rom. 10:14; 2 Thess. 1:8-9; Eph. 2:12; John 1:10-12; John 8:24; Mark 16:16; 1 Cor. 1:20-24; John 4:22; Rom. 9:31-32; Phil. 3:4-9; Acts 4:12; Eph. 5:23).” (WLC A. 60).
2) The natural man is “dead in sin, and wholly defiled in all the faculties and parts of soul and body” (WCF 6:2) including the ability to understand the truth. However, sin has not totally obliterated the capacity of unbelievers to discern Natural Law (Rom. 2:14-15; Rom. 1:19; 1 Cor. 5:1). Knowledge of Natural Law cannot make one righteous, it does not stop the unregenerate man from sinning against it and holding the truth in unrighteousness (Rom. 1:18; Jer. 17:9; Prov. 14:12).
“[Unregenerate men] hold the light of their conscience (which is as a prophet from God) prisoner. The natural man, that he may sin the more securely, imprisons the truth which he acknowledgeth, and lays hold on all the principles in his head that might any way disturb his course in sin, locking them up in restraint. Hence it appears that no man is righteous in himself, or by his own righteousness, which was the τό κρινόμενον [judge].” (John Trapp, commentary on Romans 1:18).
3) Obedience unto it cannot be considered truly good if not done A) in faith, B) according to the command of God, and C) to the glory of God. For a work to be truly good, although imperfect, the work itself must be good, and the intention of the one performing it must be good. The end of the act and the end of the actor are consistent in truly good works. Yet works are apparently good, but not truly good, when the external action is in accordance with God’s Law, but the internal intentions are not in faith nor for the glory of God; such superficial good works are “commanded by God, and are in their own nature good, but become evil by an accident, not being done in the manner, nor with the end with which they ought to be performed.” (Ursinus, Commentary on the Heidelberg Catechism, on Q. 91, p. 479).
4) It is not a complete standard for the Christian life because there are divine positive laws that God has revealed in Scripture which obligate us as well. It can be known from nature that we must worship God how he pleases rather than how we please (Acts 17:24-29), but we cannot know how God desires to be worshiped unless he reveals it to us through special revelation. Additionally, the law of nature teaches that “a due proportion of time be set apart for the worship of God,” but without divine positive law it cannot be determined which day of the week should be set apart (WCF 21:7). Natural Law teaches the duty of every nation to worship God exclusively, but the details about who God is, and how he has redeemed humanity through the Lord Jesus Christ cannot be known from nature. The duty of nations to establish the Christian religion and “kiss the Son” does not flow from natural equity per se, but rather from the positive command of the New Covenant administration. Adversely, this demonstrates the importance of Natural Law and emphasizes how the two modes of law are in harmony (cf. point 3 below).
Third, why is Natural Law important even though we have Scripture?
1) Scripture itself appeals to Natural Law and assumes its readers understand it. cf. Acts 17:29; Romans 1:26-27,32; 2:12-15; 1 Cor. 5:1; 11:14-15; 14:40; Job 31:26-28; etc. These go hand in hand, proper exegesis cannot be done without it. People may comply with what Scripture teaches in these instances, but would be unable to account for them without the law of nature that Scripture itself assumes, leaving them with some sort of Divine Command Theory, which necessarily leads down many paths to serious error.
2) The moral duties to which men are bound by nature is of use to all men generally, as well as to the unregenerate in particular, who have never been exposed to special revelation. “The moral law is of use to unregenerate men, to awaken their consciences to flee from wrath to come, and to drive them to Christ; or, upon their continuance in the estate and way of sin, to leave them inexcusable, and under the curse thereof (1 Tim. 1:9-10; Gal. 3:24; Rom. 1:20; Rom. 2:15; Gal. 3:10).” (WLC A. 96). If this were not the case, unbelievers could not be without excuse.
3) Furthermore, Natural Law teaches us to anticipate and highly esteem God’s will through special revelation. This is essentially the converse of reason 4 in the previous section. We know just enough about God from nature that we should seek him to know more, particularly how to be saved and how he is to be worshiped. It is worth emphasizing that the paramount moral duty to which mankind is bound by nature is to seek God and his special revelation. “God that made the world and all things therein…giveth to all life, and breath, and all things…That they should seek the Lord, if haply they might feel after him, and find him, though he be not far from every one of us.” (Acts 17:24, 25, 27). (cf. The Second Commandment and the Light of Nature).
4) The only way to properly distinguish A) the tripartite division of the Law, and B) the difference between general and particular equity of the judicials, is by Natural Law. This is why Theonomists, New Covenant Theology, Dispensational, and Hebrew Roots proponents greatly err on this crucial doctrine.
To summarize, if the thing that makes the law just flows from the character of God or the created order (i.e. nature) such that it was just prior to the command of God, it is a moral law. If the thing that makes it just flows from the command of God, it is a positive law. Positive laws are of two sorts, ceremonial, meaning that it pertains to the worship of God, or judicial, pertaining to the civil polity of Israel and is an enforcement of the ceremonial law. Judicial laws that enforce the moral law are also moral insofar as the circumstances are relevant; the civil magistrate, due to the 5th commandment, also has the power to institute laws of prudence and good order.
5) Scripture was given as the only rule for faith and life, not to be an exhaustive manual for civil government. There is a lot to glean from the Judicial Law, but it does not address the particulars of many things that are necessary for civil government outside of ancient Israel. This is what Proper Law, based on Natural Law, is for (1 Peter 2:13). Scripture does not even exhaustively cover every particular concerning the worship of God and government of the church, which is what circumstances of worship, based in Natural Law, is all about (cf. WCF 1:6).
6) The Light of Nature helps us distinguish between the ends for which actions exist teleologically (i.e. naturally), and the ends for which the rational person ought to intend in an action. Natural Law is the moral imperative that necessarily follows that understanding of the world. This helps us understand why some acts are sinful in themselves (e.g. sodomy, lying, etc.) and hence more egregious than others (cf. WLC 151), and, for example, why killing is acceptable in some circumstances (self defense, just war, capital punishment), but not in others (cf. WLC 136).
7) Lastly, Natural Law helps us determine whether social customs are wholesome and good, or corrupt. Scripture does not explicitly address every possible custom, it lays down general principles, but we must also use our understanding of the nature and purpose of things to determine when the particulars are appropriate. The written Word, the light of nature, and the moral imperatives that necessarily follow that understanding of nature must work together. As Calvin astutely observed:
“It is true that we must discern whether a custom is good and decent, whether it is according to nature, whether it is edifying and a good example; or if it involves corruption and vice…. This, then, is where we must begin when we speak of customs. That is, let them be (the ones) accepted by those who order their lives according to the word of God, the law of nature and human decency.” (The Importance of Customs Being Conformed to Nature | John Calvin).