For some time I have featured the work of Columbia law professor Philip Hamburger's "Separation of Church and State" with critical commentary. I just hope my criticisms are fair.The chapter to that book entitled A Theologically Liberal, Anti-Catholic, and American Principle is available online in its entirety so readers can decide for themselves if I'm being fair. I stand by my assessment; Hamburger is a brilliant scholar who meticulously documents the record, but at times weaves an utterly contentious narrative while doing so.
The "liberals" as Hamburger describes them, and as I have noted before, were either theologically unitarian or doctrinally lax in the anti-creedal, anti-clerical sense. This theologically liberal Protestantism was also arguably key to the political theology of the American Founding. Arguably, it owns a great deal of the "spirit" of the 18th Century American Founding, not just the 19th century which is the focus of Hamburger's chapter.
I've also featured the work of Dr. Gregg Frazer whose thesis describes the political theology of the American Founding as not "Christianity" or "Deism" but some kind of hybrid which he terms "theistic rationalism." One could argue that this "theistic rationalism" is actually a late 18th century version of "liberal Protestant Christianity" of the unitarian variant. Very similar to the "theologically liberal" American theologians of the 19th Century whom Hamburger tars with "animus." (Note, the 18th Century American Founders who adhered to this theology like John Adams and others also possessed such anti-RC animus.)
The legendary 19th Century Unitarian figure William Channing features prominently in Hamburger's chapter as a notable expositor of this kind of "theological liberalism." But one need not even be identifiably self consciously theologically unitarian in order to qualify as an adherent to this kind of theological liberalism. Rather, one would need to be a self consciously anti-creedal and anti-clerical Protestant. Certainly, William Livingston and John Dickinson (basically 1/2 Quaker Whigs who didn't care for creeds or clergy) would also qualify in addition to the "key Founders" that Gregg Frazer identifies (the first four American Presidents, Ben Franklin, etc.). As would the Quakers and perhaps some Baptists who also eschewed creeds. Again, lots of important figures and forces of the 18th Century American Founding.
Below is an interesting passage from page 13 of Hamburger's above linked article.
In addition, some Enlightenment Protestants attempted to reconcile religion and reason by accentuating what could be inferred from reason and by reducing religion to what was reasonable. Associating reason with the purity of their own faith, Protestants condemned Catholicism as not only unfree but also irrational and superstitious-thereby joining earlier Protestants who classed it with the mummery and horrors of paganism.
This completely resonates with the political-theological zeitgeist of the American Founding (or at least notable elements therein like the aforementioned "key Founders," Revs. Jonathan Mayhew, Charles Chauncy, and Brits. Joseph Priestley and Richard Price). But in this chapter, Hamburger apparently tries to tar it as a "bad guy" position by connecting it to animus and eventually the KKK.