So, why don't Protestants like Natural Law?
The short answer is: There isn't a short answer.
The recent history of natural law in the Protestant world ranges from its complete rejection by Karl Barth in the 1930s to the current hint of renewed interest among Protestant intellectuals. My view is that natural law is a forgotten legacy of the Reformation — one that contemporary Protestants desperately need to rediscover.
For much of Christian history, some type of natural-law theory has been used as a bridge to connect the Christian faith and culture, the church and the world. But in recent times, Protestant churches and theologians have rejected natural law as a way of showing their differences with the tradition of Roman Catholic moral theology.
The scope and unity of Roman Catholic social teaching is impressive, but without the recurrent appeal to natural law, it would lack a skeletal structure upon which to build its body of social teaching. Modern Protestant social ethics, by contrast, has no skeletal infrastructure of comparable strength. Unlike Roman Catholic moral theology, which is done in the context of the magisterial (or teaching) authority of the church, Protestant ethics has never had a “supreme court of appeals” to decide what's licit and illicit. While the Bible is the principal authority in Protestant ethics, the matter of determining “authoritative” moral teaching is complex and subject to personal interpretation. To a fault, I might add.
In his opening address at the first Christian Social Congress in 1891, the Dutch Reformed theologian Abraham Kuyper emphasized the catholicity of natural law in relation to Pope Leo XIII's new encyclical Rerum Novarum . “We must admit, to our shame,” said Kuyper, “that the Roman Catholics are far ahead of us in their study of the social problem. Indeed, very far ahead. The action of the Roman Catholics should spur us to show more dynamism. The encyclical Rerum novarum of Leo XIII states the principles which are common to all Christians, and which we share with our Roman Catholic compatriots.“
At the heart of Rerum novarum  and the recent encyclical Deus caritas est , by Pope Benedict XVI, is an appeal to reason and human nature, but not in a way that denigrates faith or revealed truth. “From God's standpoint,” insists the pope, “faith liberates reason from its blind spots and therefore helps it to be ever more fully itself. Faith enables reason to do its work more effectively and to see its proper object more clearly.” The Christian Church fulfills its responsibility to form consciences and to promote justice, when, as Benedict insists, social teaching is argued “on the basis of reason and natural law.”
In much of modern Protestant theology, there is skepticism about this appeal to reason. Protestants believe the bridge has been shattered and replaced with an ethic of divine command. So what churches and faith communities often say on social issues has no way of reaching the other side, and they end up in dangerous isolation from society and from the history of Christian moral reflection.
While Roman Catholics have held firmly to natural law, Protestants of all stripes from mainline to evangelical Lutherans, Calvinists, Baptists, Methodists, and so forth, have not. They swing between the extremes of blanket dismissal and hesitant acceptance of natural law, but even among the more favorably disposed, natural law is treated as an uninvited intruder.
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. Natural law is not merely the quest for order on the part of the state and non-Christians as Karl Barth held, it is also a profound source of truth revealed to every person — according to their capacities — through creation, conscience, and reason. 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.
Stephen J. Grabill, Ph.D., is the author of Rediscovering the Natural Law in Reformed Theological Ethics,  to be published later this month as part of the Emory University Studies in Law and Religion series from Wm. B. Eerdmans.