The book rightly focuses on theological unitarianism as an Enlightenment religion and a precursor to the more radical deism that would come later. The book properly notes denial of the Trinity as an important heterodox tenet of more Enlightened liberal religious minds of America's Founding era, naming Mayhew, Chauncy, Gay as Unitarian American preachers who so influenced America's Founders and their rational religion. He also notes America's Founders' philosophical heroes in England -- Milton, Newton, Locke, Clarke, Priestley and others -- as enlightened Unitarians. Some of these passages look like they could have been written by me -- not accusing him of anything, just noting that we draw from many of the same sources seem to think along the same track.
And so it is that I must offer my biggest criticism of the book: Wills is a political liberal and a secular leftist -- nothing wrong with that. Though, this book, like much of his work, is ideological (as some might argue all history is). The secular left are too quick to categorize too many of America's Founders as "Deists" (just as the religious right are too quick to take them as "Christians") and Wills falls prey to the same error. Here is Wills:
The reaction of the Great Awakening provided an American Unitarian boost that made Deism the religion of the educated class by the middle of the 18th century. Legal scholar William Lee Miller writes that the chief founders of the nation were all Deists -- he lists Washington, Franklin, John Adams, Jefferson, Madison, Hamilton and Paine, though many more leaders of the founding era could be added (Benjamin Rush, John Witherspoon, David Rittenhouse, Philip Freneau, Joel Barlow, Aaron Burr, James Wilson, Gouverneur Morris, Tench Coxe, to name some). Their agreement on the question of God crossed political and geographic lines. Federalist and Republican, North and South, an Adams and a Jefferson, a Hamilton and a Madison -- all were professed Deists.
Those names only qualify as "Deist" if we read the term "Deist" very broadly. (And no they didn't tend to call themselves "Deist."). John Witherspoon clearly was an orthodox Christian of the Calvinist Presbyterian bent. The kernel of truth to the claim he was "Deist" is that Witherspoon was a naturalist and a philosophical rationalist who promoted many non-Christian Scottish Enlightenment ideas. This flirtation with Enlightenment theory and philosophical rationalism could have led Witherspoon down the road to unitarianism, theistic rationalism or deistic beliefs, but it didn't; he remained orthodox.
And if we are going to read "Deism" in such a broad way, to be fair, we ought to read "Christianity" just as broadly, and if we did, all mentioned except for Paine and maybe a few others could be understood as "Christian." Tit for tat. The key Founders -- Washington, Adams, Jefferson, Madison, Franklin, Wilson, G. Morris, and Hamilton (before his end of like conversion to orthodox Christianity) were either, if read broadly, both Christians and Deists (or "Christian-Deists" as David L. Holmes puts it) or, if read narrowly, neither, but somewhere in between with rationalism as the trumping element. Witherspoon, as noted, remained orthodox. Benjamin Rush was a Trinitarian Universalist believing all would eventually be saved through Christ's universal Atonement. Paine was a strict Deist. I'm not sure about the others.