Still, it bothers me that natural law theory is so religious. You talk about God "ordaining" the natural law.
Funny that you should say that. All too often natural law thinkers are accused of not being religious enough.
Some Christians -- a minority, but a majority in some circles -- say that the only place to find moral truth is in the word of God, and that natural law tradition denies this. They argue that the natural law tradition puts much too much confidence in the capacity of fallen man to know the moral truth. They worry that the first people to use the expression "natural law" were the Stoics, who were pagans. Finally, they suspect that the God of natural law is not the God of the Bible, but the God of Deism -- a distant Creator who designed the universe, wound it up, set it running, then went away.
If I were a Christian, I might find those arguments plausible myself. How do you answer them?
Where do you want me to start?
With the first one. Since you're a Christian, why not just rely on the Bible?
For several reasons, but the best one is that the Bible itself testifies to the reality of the natural law.
Does it actually mention natural law?
It doesn't use the term "natural law," but it alludes to all the ways the law is inscribed on human nature. For example, St. Paul mentions the witness of deep conscience when he writes, "When Gentiles who have not the law [of Moses] do by nature what the law requires, ... [t]hey show that what the law requires is written on their hearts, while their conscience also bears witness and their conflicting thoughts accuse or perhaps excuse them" (Romans 2:14-15). He captures the discipline of natural consequences in the formula, "Do not be deceived; God is not mocked, for whatever a man sows, that he will also reap" (Galatians 6:7). The Bible is big on the witness of design -- design in general, design in us. And it's remarkable that when the apostles are speaking to pagans, they don't begin with scripture, but with what the pagans know already -- for instance their longing for an "unknown god," which implies knowledge that none of their deities are adequate (Acts 17:22-23).
Comment on the second criticism -- how did you put it? -- that natural law thinkers put too much confidence in the capacity of fallen man to know moral truth.
In his letter to the Christians in Rome, St. Paul doesn't blame the pagans for not having the truth about God and His moral requirements, but for suppressing and neglecting it (Romans 1:18-19). In the Proverbs, the complaint made about "fools" is not primarily that they lack knowledge but that they despise it (for example Proverbs 1:7, 1:32, 15:14.). In other words, atheism and moral obtuseness are not primarily an intellectual flaw; their most important ingredient is obstinacy. The natural law tradition does not deny these things. It recognizes error, it recognizes obstinacy, and it recognizes self-deception.
What about the third criticism, that natural law is just a baptized pagan theory?
It's true that the first philosophers to use the term "natural law" were pagans, but the biblical testimony to its reality came earlier still. Besides, if God has made some things plain through to the natural mind, wouldn't you expect the pagan philosophers to notice them? Of course their theories needed correction at many points, but that has been done.
If they were talking about the things we "can't not know," then why would they need correction?
The moral basics are one thing; the best way to describe them is another. It's only the former that we can't not know. Getting the latter right has taken centuries. The work is far from done, and our stubborn wills resist its completion.
The fourth criticism, then -- that the God of natural law is different from the God of Scripture.
No, it's an incomplete picture of the same one. Nature proclaims its Creator; Scripture tells you who He is. Nature shows you the results of His deeds in creation; Scripture tells you the results of His deeds in history. Nature manifests to you His moral requirements; Scripture tells you what to do about the fact that you don't measure up to them. Scripture is more important because it tells you the plan of salvation, but not even Scripture makes nature superfluous. It presupposes that you already have natural knowledge.