Founding era documents often invoke the concept of "Nature" (as in natural law and natural rights or the laws of Nature and Nature's God). To properly understand its usage, one must know how the Founding era defined "Nature." Put simply enough Nature = Reason. This is how the most learned modern scholarly authorities understand the concept. For instance, in Novus Ordo Seclorum, conservative historian Forrest McDonald -- who read every single document from the Founding era -- stated the Founding era defined "natural" as “discoverable by reason as opposed to revealed by God.” (p. xi.) Or as John Locke put it in his Second Treatise on Government: “The State of Nature has a law of Nature to govern it, which obliges every one, and reason...is that law....”
God may have played a role in "giving" the natural law. But nonetheless, when dealing with the rubric of "Nature," the means of discovery is man's reason. What God revealed through Nature, man's reason discovered. Or as John Adams put it:
To him who believes in the Existence and Attributes physical and moral of a God, there can be no obscurity or perplexity in defining the Law of Nature to be his wise benign and all powerful Will, discovered by Reason.
-- John Adams to Thomas Boylston Adams, March 19, 1794. Adams Papers (microfilm), reel 377, Library of Congress. Seen in James H. Hutson's, "The Founders on Religion," p. 132.
So in essence, Nature was a mechanism by which men could discover eternal, objective, immutable truths, without appealing to revelation. This doesn't mean the rubrics of nature and reason were necessarily opposed to orthodox Christianity. Some orthodox Christians believed the natural law gave an extra-biblical "confirmation" of what the scriptures already revealed. However, the concept of nature-reason had pagan origins (Aristotle) and because it was not in principle dependent on the Bible, such concept could be used, and indeed was used to attack traditional Christian orthodoxy. Deists, who disbelieved in revelation entirely, appealed "solely" to the laws of Nature and Nature's God for their understanding of Truth. And the theistic rationalists -- America's key Founders -- believed God primarily revealed Himself through Nature, partially inspired the Bible, and thus, man's reason determined which parts of the Bible were legitimately revealed.
Because of the potential conflict between reason and revelation, political philosophers who impacted America's Founding often explicated their proper roles. Most of them claimed reason and revelation by in large agreed. Orthodox Christians like Aquinas made clear that the Bible was infallible and that reason supported revelation. As noted above, the deists and theistic rationalists disagreed. Jefferson, Adams, and Franklin [and probably Washington, Madison, and many other Founders] believed Scripture was a secondary revelation, designed to support the findings of man's reason. As Franklin put it:
That Christianity, [as to it’s most essential and necessary Parts,] is plainly Nothing else, but a second Revelation of God’s Will founded upon the first Revelation, which God made to us by the Light of Nature.
Those who desire a more traditional Christian interpretation of America's Founding invariably turn to Blackstone's explication where he defined the natural law as what man discovered by his reason, distinguished between the law of nature and the revealed law, then ultimately combined them and gives "revelation" the trump.
Yet undoubtedly the revealed law is of infinitely more authenticity than that moral system which is framed by ethical writers, and denominated the natural law; because one is the law of nature, expressly declared to be so by God himself; the other is only what, by the assistance of human reason, we imagine to be that law. If we could be as certain of the latter as we are of the former, both would have an equal authority; but, till then, they can never by put in any competition together.
Upon these two foundations, the law of nature and the law of revelation, depend all human laws; that is to say, no human laws should be suffered to contradict these.
A few points in response. First, America's Founders did not view Blackstone's work as sacrosanct. Indeed, Blackstone was a Tory who believed in almost an absolute right of Parliament or the King to govern as they wished and his principles lent far more support to the British than the Americans in the Revolution. As such, Founders like Jefferson, Madison, Hamilton and others often directed harsh words against Blackstone's theories. Yet, they did have a qualified appreciation for his work. As rationalists who drew from a variety intellectual sources and synthesized them, America's Founders took from Blackstone what they thought useful and discarded the rest. And when appealing to God in the Declaration of Independence, they could have, after Blackstone appealed to "the law of nature and the law of revelation" [my emphasis]. But they didn't. They only appealed to the law of nature and of "Nature's God," both of which refer to what man's reason discovers, not what is written in the Bible. And this shouldn't surprise us given that the author of the Declaration and a majority of its drafting board [Jefferson, Franklin, and Adams] were rationalists who, unlike Blackstone, believed man's reason superseded biblical revelation.
By appealing to reason and not Scripture, the Declaration drew a very low lowest common denominator among deists, theistic rationalists/unitarians, and orthodox Christians. One can argue the appeal to reason in the Declaration makes God unnecessary. Man's reason might not be able to confirm God, but it can still discover immutable principles of human nature. And this is exactly what some modern atheistic natural law thinkers posit -- from Ayn Rand to Daniel Dennett. As Timothy Sandefur explains it:
[T]he simplistic account of law that goes by the ridiculous euphemism “realism” is absolutely inadequate, and...natural law theory does carry weight, if we will not overload it with supernaturalism and appeals to elaborate mystical structures. Many alleged natural law writers base their arguments on such things, and of course that renders their systems subjective, or runs the risk of doing just what the Pragmatist critics claimed: of misinterpreting what is really just conventional as eternal and natural. But there are certain universal factors in human life—things like “limited resources”—which are dictated by nature, and these give rise to universal rules of conduct that exist in societies, not as a matter of mere convention.
For an Objectivist like myself...natural law...is rooted not in any mystical order of the universe, but in the teleology which human nature itself reveals. The positivist argues that morality and law are based solely on convention, and are wholly subjective, and that any attempt to base rules on human nature is flawed because evolution and other modern sciences have demonstrated that human nature is malleable. The pseudo-natural law theorist, on the other hand, argues that human nature is not malleable, it is eternal, part of an unchanging universal order—and this is one reason he tends to shy away from, or even become an outright critic of, evolution. But there is “a third category of statements: those the truth of which is contingent on human beings and the world they live in retaining the salient characteristics which they have.” Natural law politics can, therefore, rest on an account of human nature, even if that nature is the product of a long-term process of evolution that is still going on.