He writes a small post on his thesis at the Immanent Frame.
The impression of great piety among the settlers is a common view of the past, probably rooted in the outsize role that the Puritans play in our mental pictures of Early America. The Puritans, however, were an odd lot in America—the exception, not the rule. (They are a prominent exception, thanks to the cultural power of their New England descendants and the voluminous records they left. One historian has complained that we “know more about the Puritans than any sane person should want to know.”)
Over the wider American landscape, however, colonists were notably “unchurched” and “un-Christian.” Scattered around in separate households (unlike the Puritans who concentrated in villages), most Americans had no church to go to and little connection to what we would call organized religion. Even where there were churches to attend, many went either irregularly or simply because the church was one of the rare places—along with the tavern—to see people in a sparsely-developed society.
Stepahnie Wolf, in her study of Revolutionary-era Germantown, Pennsylvania, estimated that only about half of the residents attended church, and that is probably a high watermark, since the community was urban and well-off, and the period was one of religious enthusiasm.
Such waves of enthusiasm (“Awakenings”) in some places and at some times rallied some people to faith, but the clergy generally despaired of the heathens who had settled the new continent. One minister trying to save souls in the American heartland in the early 1800s wrote that “. . . there are American families in this part of the country who never saw a bible, nor heard of Jesus Christ . . . the whole country, from Lake Erie to the Gulf of Mexico, is as the valley of the shadow of death.”
Most early Americans were not believers in the sense that affirming Christians are today. They were likelier to understand spells, potions, and omens than theological doctrines. Almanacs sold briskly in part because they provided guides to the occult. It took a lot of hard missionary work to displace magic with Christ.
As John Fea points out, his thesis isn't exactly novel.
Of course such an argument is not a new. Jon Butler made it in Awash in a Sea of Faith. Mark Noll, Nathan Hatch, and others have made it as well. The Founding was not a Christian event, but clearly America had become a "Christian nation" or "evangelical empire" or "a spiritual hothouse" by the early 19th century.
Regarding the argument, I'd like to see more sociological data and analysis on Church membership and attendance. It may well be that mid to late 18th Century Americans were a distinctly un-churched people, full of nominal Christians. Or, these figures may be lowballs.
I have concluded that the "Christian Nation" thesis as the evangelicals tend to promote it is bunk. Evangelicals have a tight definition for what it means to be a "Christian" (not just orthodox, but "born again," regenerate), and there is not a shred of evidence that virtually all of the Founders and populace save a handful were "Christians" in this sense. In fact, there's no evidence that a simple majority of the FFs or the populace were "Christians" in this sense. Plenty of orthodox Anglicans, for instance, would not meet this standard (neither would Roman Catholics, who are also orthodox, but much fewer in number than Anglicans in 18th Century America).
And evangelicals, especially, should understand this as they teach the narrow gate.
Were a majority of the population/FFs "orthodox" in a way that not just evangelicals, but also Roman Catholics, Anglicans could pass (for instance, following Gregg Frazer's 10 point test for late 18th Century Christianity, or perhaps an even broader sense that requires simple belief in Nicene orthodoxy, not necessarily in doctrines like original sin or eternal damnation)?
Perhaps. But there are still reasons to doubt. This kind of orthodoxy dominated religious institutions by tradition and entrenchment. But, as noted, a great deal of FFs and members of the populace were affiliated with said churches in a formal or nominal sense without believing in their official doctrines like the Trinity or that the biblical canon is the inerrant, infallible Word of God.