Natural law is the reflection of God’s moral character and the moral order of creation (sometimes referred to as the secondary law of nature) as designed by God, which is written on the human heart (Rom. 2:14-15). It is no different in substance than the ten commandments (which are a summary of the entire moral law cf. WLC 98) and even with the noetic effects of sin upon the mind of man, the light of nature is still discernible to the unregenerate (Rom. 1:18-20). Francis Turretin argues that Gentile nations rightly applying the light of nature is one criteria by which we can discern whether a judicial law is of general/common equity:

That what prevails not only among the Jews, but also among the Gentiles (following the light of right reason) is of common right. Thus the Greeks, Romans and others had their own laws in which are many things agreeing with the divine laws (which even a comparison of the Mosaic and Roman law alone, instituted by various persons, teaches).

Institutes of Elenctic Theology, XI.xxvi.3.

As Turretin alludes, even outside of Scripture, pagan philosophers and lawyers throughout history have candidly asserted that the civil government should take care to promote and countenance religion among their subjects as a necessary basis for civil order and law. With the Law of God written upon their hearts, and not entirely destroyed by the Fall, even pagans, though suppressing the truth and redirecting their worship from the living God to created things (Rom. 1:18-24), have “some scintilla, some glimmerings, dawnings, and common principles of light, both touching piety to God, equity to man, and sobriety to a man’s self” (c.f. Psalm 19:1-6, Acts 14:17, and 17:27-28; Rom. 1:18-21, and 2:12-15, 2 Cor. 5:1; Jus Divinum, pg. 9). Their idolatrous worship of that “which by nature are not gods” (Gal. 4:8), however, does not invalidate this argument any more than it would invalidate “public and social worship, stated places of meetings, ministers of religion, with many things of a similar kind, which in such a state of matters are all perverted and misapplied. Rather let us say with the prophet, ‘All people will walk every one in the name of his God; and we will walk in the name of the Lord our God for ever and ever’ (Micah 4:5)” (M’Crie, Brief View of the Evidence…).

No sentiment has been more common among all the nations than this, that it is the most important duty of those invested with public authority to pay attention to the interests of religion. The legislators and wise men among the heathen bear united testimony to this truth.[3] In the codes of law established in Greece and Rome, there were laws respecting religion, which were reckoned the most sacred and inviolable.[4] And in almost all nations, not only the civilized, but the more barbarous, ancient as well as modern, the public countenance of religion, with provision for its institutions, has formed, in one way or another, an important branch of their political regulations. These are the dictates of common reason, received and acknowledged among mankind; they are the voice of God, speaking by men of all ages and countries.
. . .
The principle is further confirmed by the consideration that religion lies at the very foundation of civil society, and that its sanctions and influence are necessary, in order to gain even the direct and immediate end of government, in the preservation of justice and peace among men. From this connection between religion and civil polity, the most enlightened writers on jurisprudence have inculcated it, as the duty of rulers, to give public countenance to religious institutions.[6]

[3] Plato asserts that “religion ought to be the principal object of care in every republic” (De Repub.). Aristotle assigns the first place among political duties to “the care about divine things,” De Polit. “The first law of the constitution (says Archytas) should be for the support of what belongs to the gods; the second, for what relates to our parents” (Apud Stobeum).

[4] The first law in the Twelve Tables of the ancient Roman institute was, “Reverence for the gods;” and by the laws of the Greeks, blasphemy, the violation of religion, etc. were made objects of punishment. Archaeologiae Atticae, p. 117.

[6] Cicero pronounces, “religion the foundation of human societies“; and shows of what great importance it is for rulers to have the people impressed with a sense of religion, to preserve religious institutions, suppress impiety, etc. (De Nat. Deor De Legib.) Plato calls religion “the bulwark of government, the bond of all society, the firmest support of legislation“; and in his book Concerning Laws, he reckons it a necessary introduction, to establish the being and providence of the gods by a law against sacrilege. “Religion” (says Plutarch) “is the first thing which claims attention in the framing of laws, for you may as easily build a city without ground, as preserve order among the citizens without a belief of the Deity. Therefore, by oaths, vows, etc., Lycurgus sanctified the Lacedemonians, Numa the Romans, ancient Ion the Athenians, and Deucalion the Greeks in general; and, by hopes and fears, kept up amongst them the awe and reverence of religion.“

Thomas M’Crie, Brief View of the Evidence for the Exercise of Civil Authority About Religion

If rulers are to govern by natural law, they ought to enforce all ten commandments, not just the last six. Many passages of Scripture approve of pagan kings enforcing the first table of the law through their understanding of natural law. The justice and equity of such judicial laws pertaining to the enforcement of the first table is not particular to Israel as a typological nation, but is common or general in its equity because is grounded in the moral law written on the heart of man. Thomas M’Crie explains,

Our argument is confirmed, by the consideration that the scripture records approved examples of magistrates who were not Jewish, who exercised their authority for the advancement of religion and the ordinances of God. We find Nebuchadnezzar and Darius publishing decrees to promote the knowledge and worship of the true God among their subjects, and prohibiting them from “speaking any thing amiss against” him (Dan. 3:29; 4; 6:26). In the books of Ezra and Nehemiah, we have an account of the edicts which several of the Persian monarchs published, in which they not only give permission to rebuild the house of God, and restore his worship in it, but positively favored and publicly countenanced the work, and supported those who were engaged in it. These were not typical kings, nor did they exercise their power in virtue of the Jewish constitution.

Brief View of the Evidence for the Exercise of Civil Authority About Religion

Even foreigners who were not a part of God’s covenanted people were subject to the sanctions of the law (Ex. 12:49) including first table offenses such as blasphemy (Lev. 24:10-16). Job, living in a pre-Mosaic nation, takes for granted that idolatry ought to be punished by the civil magistrate because it defies natural law (Job 31:26-28). False prophets and adulterers are punished by a pagan Babylonian king who was not a part of the Mosaic so-called “foedus subserviens” (Jer. 29:21-23). The first table of the law is prophesied by Zechariah to be enforced in the future where false prophets and idolaters will be punished (13:1-6). “Righteousness exalts a nation, but sin is a reproach to any people“ (Prov. 14:34) regardless if that nation is Mosaic Israel or not, therefore magistrates have the natural and moral duty to promote righteousness and punish sin according to the Law of God engraved in nature and revealed by the Holy Spirit in the Word of God.

Samuel Rutherford gives us more examples of unregenerate magistrates enforcing the first table,

That they did this [punished sins against the first table] as Princes of common equity by the law of Nature; I prove 1. Darius who was undeniably no type of Christ to his great commendation makes a Law, Ezra. 6. 11. I have made a decree that whosoever shall alter this word, let timber be pulled from his house, and being set up, let him be hanged thereon, and let his house be made a dunghill for this; and this is commended by the Holy Ghost, v. 14. They prospered through the prophesying of Haggai, etc… according to the Commandement of the God of Israel, and according to the commandment of Cyrus and Darius, and Artaxerxes King of Persia. And Ezra chap. 7. Artaxerxes saith v. 26. Whosoever will not do the law of thy God, and the law of the King (enjoining obedience thereunto) let judgment be executed speedily upon him, whether it be unto death, or unto banishment, or to confiscation of goods, or to imprisonment. And Artaxerxes was no type of Christ, yet Ezra addeth in the next verse, 27. Blessed be the Lord God of our Fathers, which hath put such a thing as this in the heart of the King, to beautify the house of the Lord at Jerusalem. If it stand good that patrons of liberty say, he was not to bless God for this, he had cause to mourn, that the heathen King being no type of Christ, should intermeddle with that which belonged not to him, to strain the tender consciences of men, and to force Religion upon them with the sword; for chap. 10. v. 7, 8. this is set down as a blessed decree which brought on an Assembly, for putting away strange wives. The like is clear in the decree of Darius, Daniel 6. for worshipping the God of Daniel, and of the King of Nineveh, for a general fast, Jonah 3. and Nebuchadnezzar, Dan. 3. 28, 29.

A Free Disputation Against Pretended Liberty of Conscience, pg. 181.

This does not at all mean that whatever law a magistrate enacts is just, certainly the above rulers also made many wicked laws and even persecuted the Church, and since then magistrates have erred greatly when they have purportedly consulted the law of nature without first and foremost examining the Word of God to ensure that they are legislating in accord with the moral law. But the fact that so many pagan rulers in Scripture and outside of Scripture have enacted godly laws according to their understanding of natural law proves that “the laws concerning moral trespass, sins against the moral law” especially against the first table, are “immutable, and common to all nations” and did not expire with the state of Israel like the laws of particular equity did (cf. Gillespie, Wholesome Severity). Magistrates are still bound to rule justly as “God’s minister,” to be a terror to bad conduct and reward good conduct (Rom. 13:1-7), to ensure that the punishment meets the crime (Deut. 19:21); and now that Christ has been given the nations as His inheritance, magistrates are obligated to “serve the Lord with fear,” “kiss the Son” (Ps. 2:11-12), and act as “nursing fathers” to the Church (Is. 49:23) until all of Christ’s enemies are put under His feet and,

“All ends of th’ earth remember shall,
and turn the Lord unto;
All kindreds of the nations
to him shall homage do:

Because the kingdom to the Lord
doth appertain as his;
Likewise among the nations
the Governor he is.“

(Ps. 22:27-28)