"Natural law was promulgated by God at creation and implanted in the human consciousness. We only know God because he has revealed himself, but he has revealed himself to us from the very beginning. Thus to say that a law is natural is to say that it is revealed in and constitutional to creation.
In the Institutes, he (Calvin) equated explicitly natural law to the Decalogue. At the beginning of his exposition he said 'that interior law' (lex illa interior) 'which we have described as written, even engraved upon the hearts of all, in a sense asserts the very same things that are to be learned from the Two Tables.' In book four, discussing civil polity, Calvin made the same point.
It is a fact that the Law of God which we call the moral law is nothing less than a testimony of natural law and of that conscience which God has inscribed upon the minds of men. Consequently, the entire scheme of this equity of which we are now speaking has been prescribed in it.
Far from being a conduit of the Classical or Thomistic view of the lex naturalis Calvin made a very sophisticated revision of the concept of natural law by removing it from the Stoic and Thomistic corpus of 'self-evident' truths and identifying it with the content of the Law revealed in the Garden and at Sinai and in the Sermon on the Mount" (R. Scott Clark, Calvin on the Lex Naturalis, Stulos Theological Journal [6/1-2, May-Nov 1998], 17-18, italics original).
"I believe it will not be out of place here to introduce the Ten Commandments of the Law, and give a brief exposition of them. In this way it will be made more clear, that the worship which God originally prescribed is still in force, (a point to which I have already adverted); and then a second point will be confirmed, viz., that the Jews not only learned from the law wherein true piety consisted, but from feeling their inability to observe it were overawed by the fear of judgements and so drawn, even against their will, towards the Mediator. In giving a summary of what constitutes the true knowledge of God, we showed that we cannot form any just conception of the character of God, without feeling overawed by his majesty, and bound to do him service. In regard to the knowledge of ourselves, we showed that it principally consists in renouncing all idea of our own strength, and divesting ourselves of all confidence in our own righteousness, while, on the other hand, under a full consciousness of our wants, we learn true humility and self-abasement. Both of these the Lord accomplishes by his Law, first, when, in assertion of the right which he has to our obedience, he calls us to reverence his majesty, and prescribes the conduct by which this reverence is manifested; and, secondly, when, by promulgating the rule of his justice, (a rule, to the rectitude of which our nature, from being depraved and perverted, is continually opposed, and to the perfection of which our ability, from its infirmity and nervelessness for good, is far from being able to attain), he charges us both with impotence and unrighteousness. Moreover, the very things contained in the two tables are, in a manner, dictated to us by that internal law, which, as has been already said, is in a manner written and stamped on every heart. For conscience, instead of allowing us to stifle our perceptions, and sleep on without interruption, acts as an inward witness and monitor, reminds us of what we owe to God, points out the distinction between good and evil, and thereby convicts us of departure from duty. But man, being immured in the darkness of error, is scarcely able, by means of that natural law, to form any tolerable idea of the worship which is acceptable to God. At all events, he is very far from forming any correct knowledge of it. In addition to this, he is so swollen with arrogance and ambition, and so blinded with self-love, that he is unable to survey, and, as it were, descend into himself, that he may so learn to humble and abase himself, and confess his misery. Therefore, as a necessary remedy, both for our dullness and our contumacy, the Lord has given us his written Law, which, by its sure attestations, removes the obscurity of the law of nature, and also, by shaking off our lethargy, makes a more lively and permanent impression on our minds" (Calvin's Institutes 2.8.1, emphasis mine).