If I may put in my two cents worth, it seems to me that just about everyone believed in Natural Law in the late Eighteenth Century.
Originally, Natural Law could be derived from Divine Law, as Richard Hooker argues. But, in fact, it descended not only from the Bible (especially St. Paul's statement that "the law of God is written in the heart" even of the gentiles), but also from Aristotle, Cicero, and the Stoics.
The Founders got Natural Law filtered through figures like Grotious and Locke. With Locke, God is pretty well removed from the picture, but you are still dealing with what has been called "spilt religion." That is, it includes cultural assumptions inherited from more religious ancestors, even though the descendents are not religious themselves and may not know where it came from.
In the case of the Founding Fathers, most of them were either Protestant Christians or Deists. Most of them either would have said that law comes ultimately from God or that, so as to keep order in society, it is best to tell the people that it comes from God. Many scholars presume that all this is not spelled out in the actual text of the Constitution because they wanted to avoid an argument.
But "Nature's God" is mentioned in what one may call the preamble, as are "inalienable rights," a phrase that has long been treated as basic to the Constitution. As scholars have pointed out, rights can only logically be inalienable if individuals lack the power to choose to relinquish them, and that can only be the case because they are God-given and God-maintained. Thus you could voluntarily sign a document putting yourself into slavery, but because the right to liberty is inalienable, that document could not have constitutional force.
Keep in mind that during the colonial crisis over Parliament's overextension of its powers in America (such through the tax acts, primarily), there's no record of it having occurred to anyone, on either side of the debate, to have a court resolve the question. It was understood that questions of natural law were to be resolved by the people themselves.
I have three questions for inquest, if he'd take them on, I'd be interested in his take on them. (1) What laws does the Supreme Court apply in federal territories and districts -- or do they have their own laws homegrown on territorial or district soil? (2) Does the Supreme Court rule on disputes between states, or on disputes between individuals in the states? That is, is the individual or the state standing before the court? (3) What was the majority opinion in Dredd Scott and is that good law?