They relied on the excellent historical work of Gary Scott Smith chair of the history dept. at Grove City College and one of the most prominent evangelical historians. Notice their classification of the first half dozen:
PRESIDENT TERM DATES DENOMINATIONAL AFFILIATION
George Washington 1789-97 Episcopalian (theistic rationalist*)
John Adams 1797-1801 Congregationalist; Unitarian
Thomas Jefferson 1801-09 Episcopalian (theistic rationalist*)
James Madison 1809-17 Episcopalian (theistic rationalist*)
James Monroe 1817-25 Episcopalian (deist?)
John Quincy Adams 1825-29 Unitarian
Andrew Jackson 1829-37 Presbyterian
Here is what they put next to the "*":
*The term "deist" is often used for a number of early presidents and founding fathers, though this causes confusion. For some of these founders, historian Gary Scott Smith prefers the term "theistic rationalism," which mixed elements of natural religion, Christianity, and rationalism, and relied foremost on reason. Unlike deists, theistic rationalists believed that God was active in the world and that prayer was therefore effectual. They contended that religion's primary role was to promote morality, which was indispensable to society.
What's striking is according to Christian Nationalist standards, the first "Christian" President was probably Andrew Jackson. Traditional Christians define Christianity fairly narrowly according to orthodox Trinitarian doctrines. Because the "theistic rationalists" weren't orthodox Trinitarians -- yet because they also weren't "Deists" -- a new term was needed. Theistic rationalism is similar to John Adams' "Unitarianism" in all but name only. It's basically Christianity stripped of all of its orthodox doctrines such that theism and mere morality remain.
Glad to see there are honest evangelicals who get it. Indeed, they were among the first to get it.