Being fully committed to the Protestant Reformed tradition--especially as it is represented at Westminster Theological Seminary--I have developed a basic understanding of natural law theories over the years. If by "natural law" we mean a moral order that is (a) revealed by God in nature, (b) stands behind conscience, (c) obligates all people to worship and obey Him, and (d) is sufficient to leave all without excuse and liable to divine judgment for sin, then I affirm it. However, one standard theistic account of natural law (NL) as a moral theory goes further. This account claims that all people can not only apprehend certain moral truths by unaided reason - apart from biblical revelation - but that people can, in principle, espouse and properly act upon those truths, again, apart from saving grace. It's this feature of NL theory--perhaps the critical feature, it seems to me--that allegedly opens up "common ground" for Christians to cooperate with people of other faiths (or of no faith at all) on issues pertaining to the "common good."
Now, I have learned to leave the majority of negative assessments to my colleague and resident pessimist, Carl Trueman. But I must say that, from a Reformed perspective, this additional claim by many Natural Law theorists runs into a number of obstacles. I wish to briefly mention two.
I believe this aspect of the Natural Law theory in view--that people can reason their way to actionable truths apart from God's special revelation--is too optimistic about the powers of unaided reason after the fall. The general revelation of God in nature and beneath conscience must be "carefully distinguished from the reaction that sinful man makes to this revelation" (Van Til). The apostle Paul says that unbelievers "suppress the truth" that they know (including the truth of their moral obligation to God), that they are, at root, "hostile to God" (Rom 8:7); that they have become "futile in their thinking" (Rom 1:21). They are, Paul says elsewhere, "darkened in their understanding, alienated from the life of God because of the ignorance that is in them, due to their hardness of heart" (Eph 4:18).
These are hard words, no doubt. But they point to one side of what has been called the "antithesis" between belief and unbelief, a moral and spiritual conflict of basic commitments that touch all that Christians and non-Christians think about and discuss. According to this Scriptural principle, fallen man is slavishly committed to his own moral autonomy, while Christians are to view all things under the Lordship of Christ and the light of His Word. This means that, at the deepest level, there is no mutually acknowledged common ground between Christian and non-Christian. And this, it seems to me, leaves NL proponents calling for peace when there is no peace.
This is not to deny that by God's common grace, many unbelievers are immensely gifted and do morally upright things--often outstripping many Christians in good deeds. But such acts do not spring from an essentially unfallen rational ability, in principle, to discern and apply precepts of natural law. Rather, it is God who mercifully restrains the unbeliever's hostility against Him, so that the unbeliever is led, to some degree, to live inconsistently with his moral depravity. So common grace may facilitate a kind of formal agreement between the Christian and non-Christian. But common grace remains just that--grace. God gives it when and where He wills. You can't count on it as a foundation for public policy. This is a second reason why, I think, the NL theory I have in mind is a non-starter for programmatically advancing public morality.
To close on a positive note, Christians should confidently reason from Scripture in all of life, including life in the public square--rather than appeal to fallen unaided reason. We should do it because failing to do it leads, at best, to what we could call various forms of "well-articulated pragmatism." We should do it because God designed for us to read His general and special revelation together, never to separate the two. But Christians should reason from Scripture, above all, because it is there that we meet the Christ in whom are hidden "all the treasures of wisdom and knowledge" (Col 2:3), including wisdom for the public square. Such a Christ-centered theology for the public square, I think, better comports with what God says to us, and does not depend on what we say to ourselves.
*This post is a slightly revised version of the opening remarks Dr. Wynne offered during a panel discussion on natural law at a "Faith in the Public Square" conference at Westminster Theological Seminary in October 2016.