The Decalogue and the Natural Law
Thomists asserted that the Decalogue expressed the essence of the natural law:
Man in the natural state is fully able to perceive and follow the natural law, since he is a rational being created in the image of God. For this he needs no divine revelation. So much the Thomists did not hesitate to affirm. But as well they admitted that because of the Fall, the will was corrupt to the extent that the good which the natural man perceived he could not follow; more than this, his conception of the natural law was itself corrupted. Hence the need for the divine revealed law. This law was contained in the Old and the New Testaments, and received its summary declaration in the Decalogue.
Skinner notes that the Thomists made two important connections between the natural and the divine law. The first is that though Christ’s coming released man from bondage to the Old Testament law, it did not release him from obedience to the law of nature, but rather more firmly wrote the law in the heart of the believer, to enable him to obey it even more. Secondly, the Old Testament law did not lose its place as the divine transcript of the natural law; the Mosaic Law is known to represent the will of God and so is the expression, although not the complete expression, of the natural law. Therefore the Decalogue must be made the basis of all other laws. “Neoscholastic political thinking … is firmly rooted in the assumption that the validity of the Old Covenant – in reality, as these publicists readily admit, nothing more than the Hebrew version of natural law – is eternal, and that for the purposes of political affairs … the coming of Christ made no difference whatsoever.” [Calvin and the Whigs, pp. 48–49.]
Calvinism likewise asserted this equivalence, as exemplified in Althusius:
XXI. §29. The Decalogue expresses the universal law of nature. Even though it was given to the Jews, “the Decalogue has been prescribed for all people to the extent that it agrees with and explains the common law of nature for all peoples [lege naturae omnibus gentibus communi]. It has also been renewed and confirmed by Christ our king.” [Althusius, Politica Methodice Digesta, in The Debate that Changed the West, p. 152.]
Interestingly, Hugo Grotius likewise initially accepted this equivalence:
As a patriotic Dutchman, Grotius also accepted and vociferously defended major elements of the Calvinist political synthesis. This is very clear from a perusal of his De Jure Praedae Commentarius, “Commentary on the Law of Prize and Booty” written in 1604. The spirit in which this was written is comparable to the Huguenot tracts, and is remarkably similar to that of Johannes Althusius’ masterpiece of Calvinist political theory, Politica Methodici Digesta, “Digest of Politics Methodically explained,” first published in 1603. Here Grotius is the staunch advocate of the rights and liberties of the realm, and of the role of the estates as defenders of those rights. As the theoretical basis for this arrangement he, like Calvinists in general, appealed to the traditional concept of the natural law as the expression of the will of God, revealed in the Decalogue, and expressed exemplarily in the Old and New Testaments. In Calvinist fashion, Moses is given pride of place as the pre-eminent example of justice for modern statesmen to follow, and the Old Testament is repeatedly referred to for political norms. [Calvin and the Whigs, pp. 85–86.]
But later on Grotius became a leader of the Arminian opposition in the Netherlands. He likewise abandoned this position to develop a basic tenet of humanistic natural law, that the Ten Commandments are a species of positive as opposed to natural law:
§ XV–XVII. Divine positive law is given either to all mankind or to a single nation. It was given three times to the human race: at the creation, after the Flood (to Noah), and in Christ, and it is binding once it has sufficiently been made known. Hence the Law of Moses and specifically the Ten Commandments are peculiar to the nation of Israel and are not binding on the nations. The Mosaic Law still has some use for the Christian, because whatever it enjoins cannot be contrary to the law of nature. [Grotius, De Jure Belli ac Pacis, Book I, ch. I, in The Debate that Changed the West, p. 210.]
Bottom line: Both Thomists and Calvinists asserted that the Decalogue expresses natural law, while Hugo Grotius, the founder of modern humanistic natural law, denied that position.
 See MacIntyre, Whose Justice? Which Rationality?, chs. X-XI, for Aquinas; Carlos López Hernández, Ley, Evangelio, y Derecho Canonico en Francisco de Vitoria [Law, Gospel, and Canon Law in Francisco de Vitoria] (Salamanca: Universidad Pontifica de Salamanca, 1981), which develops the Thomist understanding of law and grace, in both Aquinas and in Francisco de Vitoria, by contrast with the Lutheran.
 Skinner, Foundations, vol. 2, pp. 150–1.
 J.A. Fernández-Santamaria, The State, War, and Peace: Spanish Political Thought in the Renaissance, 1516–1559 (Cambridge, England: Cambridge Univ. Press, 1977), p. 122.
 “In his first writings, his youthwork, Grotius is completely pervaded with the classical writings of which he had studied and made comment. Then he came steadily more in contact with the ideas of his own time, of the right of resistance and popular sovereignty, as the Monarchomachs [Calvinists] proclaimed them and as they were also to be found in the Netherlands. These and those of Antiquity Grotius took up and assimilated.*** Grotius assimilated the concepts of Antiquity, of the theocratic Middle Ages, the Monarchomachs and the absolutists of his time.” Dr. A.C.J. de Vrankrijker, De Staatsleer van Hugo de Groot en Zijn Nederlandsche Tijdgenooten [The Doctrine of State of Hugo Grotius and his Dutch Contemporaries] (Nijmegen: Dekker & van de Vegt, 1937), pp. 14–15, 22. This book provides an excellent analysis of Grotius’ political works in comparison with contemporary Calvinist theory.
 Hugo Grotius, Commentary on the Law of Prize and Booty, ed. and with an Introduction by Martine Julia van Ittersum (Indianapolis: Liberty Fund, 2006).
 See Barbeyrac’s note, p. 167 of the Liberty Fund edition.