I remarked previously that Irenaeus considers the content of the natural law and the Decalogue to be identical.
This is pretty standard stuff generally and is the standard position of the Reformed tradition. Turretin, for instance, teaches it in his Institutes of Elenctic Theology.
First, Turretin describes the moral law as “the pattern of God’s image in man” and as a “copy or shadow” of God’s archetypal law. It thus has the character of immutability: nothing can change its applicability to the regulation of human life.
Fourth, the moral law (which is the pattern of God’s image in man) ought to correspond with the eternal and archetypal law in God, since it is its copy and shadow (aposkimation), in which he has manifested his justice and holiness. Hence we cannot conform ourselves to the image of God (to the imitation of which Scripture so often exhorts us) except by regulating our lives in accordance with the precepts of this law. So when its observation is enjoined, the voice is frequently heard, “Be ye holy, for I am holy.” Now this law is immutable and perpetual. Therefore the moral law (its ectype) must necessarily also be immutable. (11.2.16)
Next, he equates this moral law, founded on God’s eternal law, with the “natural” law. All of its precepts, he says, are “taught by sound reason.” Since it is founded on man’s rational nature, it is universally binding on all peoples and nations. Furthermore, as long as human nature remains the same (and it “is always the same and like itself”), it is perpetually binding. Viewed in this light, the substance of the Decalogue, which also functioned in the Mosaic administration, is not limited to that administration. In design, it remains in force as regulating the lives of everyone, believer and unbeliever alike.
Fifth, the moral law is the same as to substance with the natural, which is immutable and founded upon the rational nature; both because the sum of the law (which is exhausted by the love of God and of our neighbor) is impressed upon man by nature and because all its precepts are derived from the light of nature and nothing is found in them which is not taught be sound reason; nothing which does not pertain to all nations in every age; nothing which is not necessary for human nature to follow in order to attain its end. Therefore it ought to be of perpetual right because the rational nature is always the same and like itself. Hence what is founded upon it must also be such. If by the sin of man the rational nature was changed in the concrete and subjectively, the law was not forthwith altered in the abstract and objectively. (11.2.17)
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