John Fea's website pointed me to this, a series of letters where Francis Schaeffer argued the Christian Nation thesis with Mark Noll and George Marsden. From Fea:
Second, Schaeffer was a leading force behind the emergence of the Christian Right in the late 1970s and early 1980s and was influential in promoting the idea that America was founded as a Christian nation.You can preview this section of Hankins' book on Googlebooks.
It is on this latter point that Hankins really shines. His chapter on this subject is worth the price of the book. Hankins had access to a series of letters written between Mark Noll, George Marsden, and Francis Schaeffer on the topic of whether or not America was a Christian nation. Schaeffer chided Marsden and Noll for showing too much respect for secular scholarship and Marsden and Noll tried to convince Schaeffer that his view of the founding was utterly wrong. These letters provide the context for Marsden and Noll's book (with Nathan Hatch) The Search for Christian America.
This correspondence took place mainly in 1982. It's interesting, without knowing the content of the correspondence, I've already concluded some of the same points as Noll and Marsden. I've nailed the late Schaeffer numerous times on his, on the one hand, support for the "Christian Nation" thesis, and on the other, his dismissal of Aristotelian-Aquinas thought as authentically "Christian." Read page 217 if googlebooks lets you. The political theology of the American Founding typified "Nature" substituting for "Grace," something Schaeffer railed against and Noll and Marsden let Schaeffer know it.