I'd be remiss if I didn't note Dr. Kuznicki's post which Andrew Sullivan noticed and described as "very elegant." I especially liked where Dr. Kuznicki informs on the attributes of Nature's God:
To the founders, nature's God was the deity of every religion -- and of none. Nature's God was present wherever religionists of any faith showed decency and kindness toward their fellow man; nature's God was absent when the faithful were cruel, intolerant, or uncharitable. Nature's God demanded that every one of us come to Him on our own terms, not under threat of compulsion. Why not? Because it is impossible to imagine a God who wanted compelled, inauthentic, grudgingly given prayers.
As the founders meant it, nature's God can be the deity of anyone who believes in God. Even atheists can believe that human nature, stripped of the deity, still demands a sincere conscience, free of all compulsion, as the foundation of any legitimate faith, or civil society, or government. Even atheists can believe, as it were, in nature's God. It's the one thing we all can agree on, because sincere dialogue, with no imposture or compulsion, is the prerequisite to any good spiritual aim we might have, and because a religion based on force cannot possibly be a good one.
This importantly understands how the principles liberalism connect with "the civil religion." Many on the secular left don't take any metaphysical underpinnings of civil government seriously, dismissing the Declaration as mere "rhetoric." And the religious right tries to "co-opt" its theological principles as "Christian" or "Biblical." (Jon Meacham's book discusses this in detail).
The Declaration and its metaphysical claims were remarkably pluralistic for the time, as were the Founders. This shouldn't surprise given the Founders' personal "theistic rationalist" creed posited that most if not all religions, even the Pagan and Eastern ones, taught the same basic Truth as Christianity and were thus valid paths to God (The end of Gregg Frazer's Ph.D. thesis notes how the personal faith of the Founders connects with the ideas of our Founding documents). Indeed I can quote John Adams and company where they note Nature's God is found within not only Christianity, but Hinduism, Pagan Greco-Romanism, Native American spirituality, etc. (See this post where Dr. Frazer examines one of John Adams's letters to Jefferson.)
Now, as some have pointed out, their theology may not have been sound (do all world religions really teach the same Truth?) and the metaphysic behind the Declaration may be unprovable, but the Founders' formulation did lay very solid ground for the Founding of liberal democracy in general, the United States in particular. Given that liberal democracies produce for those nations which embrace its ideals, an abundance of, in Allan Bloom's words, "peace, gentleness, prosperity, productivity" and, I might add, "pluralism," I believe such ideals, in the abstract, are worth defending with a religious zeal, as though they were the Gospel, regardless of whether they can be falsified in a scientific hypothesis as such.