There has long been confusion about whether natural law is an acceptable basis for Christian thinking or a Christian view of law. The Declaration of Independence, for instance, equated natural law and God's law when it referred to "the laws of Nature and Nature's God." Jefferson's confusion was partly the rhetoric of a man who was respectful of, but distant from, the Christian faith and partly from the confusion amongst moral philosophers of the era.
Eighteenth century natural law philosophers, and others before them, treated feudal society as a norm rather than an outgrowth of a real, if imperfect, Christianization of society and its relationships in terms of Biblical law. Thus, God's law became a given and hence "natural." Such thinking is not only non-historical, but also distinctly non-Biblical.
Scripture tells us that nature is fallen and man depraved by a sin nature. Nature is not to be seen as a source of law or revelation. Only God is true and only His revelation is law. His creation may reflect His law, but is not a source of it. If nature is the, or even an, independent source of law, then man is its mouthpiece. Natural law is an open invitation to the autonomous mind of man interpreting nature as law.
In a more Christian era, it was easy to see the prevailing ethic as "natural." It was not natural; it was the moral capital of a Christian culture which had self-consciously limited state authority after the fall of Rome, the last great pagan empire of antiquity.
Law does not come from nature; law comes from the Creator of nature. In the physical realm "the laws of nature" are a mis-named reference to God's established laws over the material creation. Likewise, the reference in moral philosophy to "natural law" credits nature as the self-evident source of ethics and law.
Much has happened since the eighteenth century use of the term "natural law." Humanists have become more self-conscious about applying their philosophy, and will not allow God's law into moral philosophy any more than they will allow it into biology classes. If a humanist sees law in nature it is because the mind of man decrees it to be so. In addition, Darwin redefined nature as a random realm of chance. Darwin destroyed the non-Christian's belief in nature as a realm of law and substituted the rational scientist as the interpreter of nature. Modern natural law theorists are thus humanistic, though sometimes conservative humanists. Natural law is, in reality, used as an alternative to God's law, not its equivalent.
If law comes from nature, God's revealed law is depreciated. If law comes from God there is no natural law, only God's law very imperfectly reflected in a fallen world. If moral law comes to man by the revelation of God in Holy Scripture, we do not need to appeal to the fallen world of nature as a substitute revelation. Natural law is a false source of law because nature is a false god. It is time Christians stood for God's law because it is God's law. If the Word of God is insufficient to persuade men of moral absolutes, they will not be persuaded by an ambiguous standard such as natural law.