This NY Times Sunday Magazine article strains to get it right but doesn't quite get there. Using the Texas textbook adoption controversy as a hook, author Russell Shorto reports on the Christian Right's attempt to link the Declaration of Indendence (with its multiple references to God, in various forms) and the Constitution. Shorto (and probably many of the activists he interviewed) could have noted the pairing of American time and Christian time at the end of the original Constitution. It is retained even today in presidential proclamations, as in this latest one, for American Heart Month: "done in the year of our Lord 2010." If one protests that this expression was merely a convention of the time, that actually strengthens rather than weakens the Founding as Christian argument: Conventions such as that have weight in our original understanding of the text.
One should consider the constitutional commands ("shall") that public officials shall take oaths to support the Constitution but that "no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States." This summarizes the issue well: There was a public expectation of reverence or piety (the oath requirement) but without demanding a sectarian commitment.
Contemporary secularists have grotesquely expanded the private sphere to shove religion out of the public sphere. As religion fights its way back, its adherents need to consider all of the language and argument of the Declaration of Independence and thus unite spirit and reason.