While the quote doesn't appear in any of Barton's later works, it does turn up in another popular Christian book, William J. Federer's, America's God and Country: Encyclopedia of Quotations, p. 18. Federer provides a date for the quotation (July 4, 1821), and gives the source as follows:
We recently located this source and now suspect that John Quincy Adams never uttered these words. Here's what we found:
Pages X through XXXVIII of the Thornton book are a historical introduction to the subject of religion in the New England States, with a special focus on the state of Massachusetts. Throughout this introduction, Thornton quotes various early Americans on the subject of religion. At least some of the quotations are footnoted, and all of them appear to be enclosed in quotation marks. Sometimes portions of the quotations are italicized for emphasis.
The words attributed to John Quincy Adams appear on page XXIX. None of these words are placed in quotation marks. Rather, the sentence reads as if Thornton is making his own conclusion about what John Quincy Adams believed. Thornton's sentence reads as follows:
No footnote for these words is given. Nor are the words attached to a date. Hence, if these words are a quotation from Adams, it is impossible to trace them back from Thornton's book to an original source. Elsewhere in the book Adams' father (John Adams) is quoted properly, i.e., with footnotes and quotation marks.
It appears, in other words, that somewhere down the line Thornton's conclusions about John Quincy Adams were passed off as Adam's own remarks. In Federer's case, his reproduction of the quotation doesn't edit out the words "said John Quincy Adams" and replace them with ellipses; either he knowingly misreports Thornton's words, or he didn't check his sources for accuracy. It is, of course, possible, that the printer made a mistake and forgot the quotation marks but, until somebody can locate the original source of the quote, there is no ground whatsoever to treat these words and Adams' own. The quote should be regarded as bogus.
Please note: even if Adams did say these words it wouldn't bolster the accomodationist's case; as we suggest elsewhere, Adams would simply be wrong to argue that the federal Constitution embodies the principles of Christianity. It doesn't, and Adams' saying so doesn't prove a thing. Rather, the real importance of this quote is as a demonstration of just how far some popular Christian authors will go to prove their case. Nothing in the Thornton book justifies taking the "indissoluble bond" quote as John Quincy Adams' own words, but because it says something the right wants to hear, the words are pressed into service anyway. This isn't good scholarship, and the consumers of Barton and Federer's work should be aware of just how poor their research is.