As to the whole counsel of God, the Westminster Divines teach that it provides all that we need for the life of man to be lived to God's glory, and yet some things "common to human actions and societies,... are to be ordered by the light of nature and Christian prudence, according to the general rules of the word, which are always to be observed" (WCF 1.6).
The whole counsel of God, then, does not exclude the application of the light of nature and Christian prudence in the regulation of what is appropriate and common to human societies, but provides general rules which are always to be observed, even by Christian magistrates.
To explain further, I wrote the following in a chapter called "The Puritan Legacy Considered" at the conclusion of an essay by Dr. Steven Dilday on Postmodern Skepticism, Relativism, and Religious Toleration in the Light of the Westminster Standards and the Thought of George Gillespie, found in Steven Dilday, Two Essays on the Thought of George Gillespie (2009, ed., R. Andrew Myers), pp. 48-49:
And finally, this Biblical paradigm [the Puritan view of magistracy] is theodidactic,4 allowing for the employment of magisterial wisdom and discretion grounded in the whole counsel of God, rather than being confined to a blanket application of Mosaic law. As Gillespie noted:
I know some divines hold that the judicial law of Moses, so far as concerneth the punishments of sins against the moral law, idolatry, blasphemy, Sabbath-breaking, adultery, theft, etc., ought to be a rule to the Christian magistrate; and, for my part, I wish more respect were had to it, and that it were more consulted with.5
The judicial laws of Moses are indeed of great use to Christian magistrates today,1 being part of the whole of Scripture, all of which is "profitable for doctrine, for reproof, for correction, for instruction in righteousness" (2 Tim. 3:16); and, although they are no longer nomos/law, having "expired with the state of Israel" (WCF 19:4), they are still didache, that is, part of the teaching of the Word of God.
4. The contribution of Rev. Matthew Winzer to the understanding of this term is gratefully acknowledged.
5. Ibid [Aaron's Rod Blossoming], 2.
1. Judicial laws, grounded in moral equity, "remain as good directions to order even Christian politics accordingly." William Gouge, Commentary on Hebrews (London, 1655; Birmingham, AL: Solid Ground Christian Books, 2006, reprint of 1980 Kregel Publications ed. and 1866 Nichols ed.) vol. 1, 505.
That right discernment of judicial laws, as to what of general equity binds Christian magistrates today and what of that which was peculiar and temporary with respect to a nation whose polity has ceased does not, requires intimate acquaintance with the whole of counsel of God, goes without saying. Like Joshua, the Christian magistrate ought to "meditate...day and night" upon the whole of God's Word, including "the book of the law" that he might rule with wisdom and good success (Jos. 1.8).
That discernment is reflected in the Reply of Thomas Cartwright to the Answer to the Admontion to Parliament, as quoted by Archbishop John Whitgift in his Works, Vol. 1, p. 270:
And, as for the judicial law, forasmuch as there are some of them made in regard of the region where they were given, and of a people to whom they were given, the prince and magistrate, keeping the substance and equity of them (as it were the marrow), may change the circumstances of them, as the times and places and manners of the people shall require. But to say that any magistrate can save the life of blasphemers, contempuous and stubborn idolaters, murderers, adulterers, incestuous persons, and such like, which God by his judicial law hath commanded to be put to death, I do utterly deny, and am ready to prove, if that pertained to this question. And therefore, although the judicial laws are permitted to the discretion of the prince and magistrate, yet not so generally as you seem to affirm, and, as I have oftentimes said, that not only it must not be done against the word, but according to the word, and by it.
And in Cartwright's Second Reply he adds:
It is not that the magistrate is simply bound unto the judicial laws of Moses, that that he is bound to the equity, which I also called the substance and marrow of them.
Francis Turretin provides guidelines for the exercise of this discernment likewise, Institutes of Elenctic Theology, Vol. 2, pp. 165-167:
Whether the judicial law was abrogated under the New Testament. We make distinctions.
III. In that law various ends must be distinguished....Undoubtedly those things are to be accurately distinguished which in the law were of particular right (which peculiarly applied to the Jews in relation to time, place and Jewish nation: such was the law concerning a husband's brother, the writing of divorcement, the gleaning, etc.) from those which were of common and universal right, founded upon the law of nature common to all (such as the laws concerning trials and the punishment of crimes widows, orphans, strangers and the like, which are of moral and common right). As to the former, they may well be said to have been abrogated because the Jewish polity having been taken away, whatever had a peculiar relation to it must also necessarily have ceased. But as to the latter, it still remains because it enters into the nature of the moral and perpetual law and was commanded to the Jews not as Jews simply, but as men subject with others to the law of nature. For distinguishing those things which are of common and particular right, a threefold criterion can be employed. (1) 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 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). (2) What is found to be conformed to the precepts of the decalogue and serves to explain and conform it. This is easily gathered, if either the object and the matter of the laws or the causes of sanctioning them are attended to. (3) The things so repeated in the New Testament that their observance is commended to Christians.
The Christian magistrate, therefore, as a minister of God (Rom. 13.4) -- like the minister of the Word who must rightly divide the word of truth (2 Tim. 2.15) -- must be informed by the Word of God, which generally is his rule, as it is of all Christians, according to the moral law; and as the judicial law is of special interest to magistrates as a divine exemplar, so it is to be referred to and "consulted with" as they provide "good directions to order even Christian politics accordingly," with appropriate distinctions made according to the "light of nature and Christian prudence." Hence, as the Apostle says, "For whatsoever things were written aforetime were written for our learning, that we through patience and comfort of the scriptures might have hope" (Rom. 15.4), and thus the judicial law too may teach us, and is therefore theodidactic, that is, not nomos, but didache.