Nominal Christianity in Early America?

In light of all the recent books by Sam Harris, Richard Dawkins, Daniel Dennett, Christoper Hitchens, et al, on the nature of religion and in some cases its particular manifestation in America, I found this tidbit from Sean Wilentz’s “The Rise of American Democracy” quite interesting:

Although the figures are sketchy, it appears that as few as one in ten Americans were active church members in the unsettled aftermath of the American Revolution. The evangelizing that ensued proceeded, at first, in fits and starts, but gathered tremendous momentum after 1825. By the 1840s, the preponderance of Americans - as many as eight in ten - were churched, chiefly as evangelizing Methodists of Baptists (in the South) or as so-called New school revivalist Presbyterians or Congregationalists (in the North.) What was, in 1787, a nation of nominal Christians - its public culture shaped more by Enlightenment rationalism than Protestant piety - had turned, by the mid-1840s, into the most devoted evangelical Protestant nation on earth. (p. 267)

This is a highly recommended book chronicling the evolution of democracy and the rise political parties in the U.S. from the end of the Revolutionary period up to the Civil War. Wilentz’s technique of using footnotes only at the end of large blocks of text makes it difficult to track down exactly where he obtained the figures mentioned in the paragraph above.