As John Fea titles it. I've "discovered" a lot about history too. And, like Cameron I've noticed a lot of "rewriting" -- a lot of it from HIS side. I'll be watching his documentary very carefully and reporting on it. Now, we should understand that poisoning the well is a logical fallacy (the genetic fallacy). So if a fervent non-Christian secular leftist like Chris Rodda or PZ Meyers accurately points out an error from David Barton or Kirk Cameron, they accurately point out the error. Though orthodox Christians may have serious issues with the ultimate worldview of the "sources," facts are facts and logic is logic.
But even if evangelical orthodox Christians look for "trustworthy" sources (that is humoring the genetic fallacy that dismisses anything secular leftists say), smart, indeed brilliant, scholarly sources of the orthodox Christian bent whose "biblical" and "fundamentalist" bona fides are beyond reproach dispute Cameron's "Christian America" narrative. Indeed, according to an orthodox biblical perspective, arguably "Christian Americanism" is a heresy not unlike Mormonism. "Christian Americanism" adds a third sacred holy book -- Americanism -- to the Old and New Testament. A "Holy Book" -- like the Book of Mormon -- that purports to perfectly comply with what is written in the OT and NT, but arguably contradicts or inappropriately adds to them.
Numerous valuable scholarly sources written from the perspective of orthodox Christianity exist to keep "Christian Americanism" is check; but I'll name three that I think we should use when "testing" Cameron's argument. In order dated earliest published: First, "The Search For Christian America," by Mark Noll, Nathan Hatch and George Marsden; second, "Was America Founded As a Christian Nation?: A Historical Introduction," by John Fea; and third, "The Religious Beliefs of America's Founders: Reason, Revelation, Revolution," by Gregg Frazer.