The “nature” referred to in natural law can mean different things, but I mean by it the divinely engrafted knowledge of morality in human reason and conscience, that which all human beings share by virtue of their creation in God’s image. Theologically speaking, I think this understanding of nature points back to our original creation in God’s image, but it also anticipates the fall into sin, where the divine image was corrupted but not destroyed.
“Law,” too, can vary in meaning, but we have used it here as shorthand for the universal moral law written into the human heart by God. Law as a representation of God’s will can be known through a variety of means such as the Ten Commandments, the Torah, the Sermon on the Mount, the pangs of conscience, or the rational intuition of good and evil. When “nature” and “law” are understood in these ways, the claim that natural law is a forgotten legacy of the Reformation is certainly an understatement.
Natural law holds great promise as a bridge to connect the Christian faith to culture, although from the fuller perspective of God’s revelation in Jesus Christ, natural law has limited but significant value. . . . When natural law is understood properly, only so much should be expected from it as a source of revelation. God does not save the world through natural law, nor does he reconcile the world through the pursuit of justice; but he does provide a public record of his eternal power and divinity through the law written on the heart.
To explore this important subject further, see Gabrill's blog (Uncommon Truths), especially his series on why Protestants don't like natural law.
It seems to me that a neglected aspect of the natural law debates concern the relationship between Reformational theology and the natural law tradition. Some Reformation historians, like John T. McNeill (the editor of Calvin’s Institutes) sees a strong line of continuity:
There is no real discontinuity between the teaching of the Reformers and that of their predecessors with respect to natural law. Not one of the leaders of the Reformation assails the principle. Instead, with the possible exception of Zwingli, they all on occasion express a quite ungrudging respect for the moral law naturally implanted in the human heart and seek to inculcate this attitude in their readers. Natural law is not one of the issues on which they bring the Scholastics under criticism. With safeguards of their primary doctrines but without conscious resistance on their part, natural law enters into the framework of their thought and is an assumption of their political and social teaching. . . . The assumption of some contemporary theologians that natural law has no place in the company of Reformation theology cannot be allowed to govern historical inquiry or to lead us to ignore, minimize, or evacuate of reality, the positive utterances on natural law scattered through the works of the Reformers. . . . For the Reformers, as for the Fathers, canonists, and Scholastics, natural law stood affirmed on the pages of Scripture. "Natural Law in the Teaching of the Reformers," Journal of Religion 26 (1946), p. 168.For more on this connection, see Gabrill's book, Rediscovering the Natural Law in Reformed Theological Ethics, as well as Westminster Seminary's David Vandrunen's A Biblical Case for Natural Law (75 pp.) I have not yet read either book, though I plan to do so soon!