I've been reading L. C. Rudolph's 1963 book, Hoosier Zion: The Presbyterians in Early Indiana (full text available at Internet Archive, archive.com, if that works better for you than a physical copy).
It's not just about religion and not just about Indiana. Rudolph is writing from a particular angle about the culture clash that created the Midwest among other places -- between Yankees and Southerners. (Of course both terms have to be broadly defined, since Scotch-Irish folks might appear on either side.) The author has enough distance to tell the story from both sides, and a narrow enough focus to keep it close to the ground. Indiana was the most difficult non-slave state for the Presbyterian missionaries, so in some ways it makes the best story. It's worthwhile just to remember that there was a time, around 200 years ago or a little less, when Americans were very concerned about what sort of society was going to emerge in the "West" and that would presumably dominate the country.
"Exotic" is the literal word for Presbyterian ministers in early Indiana. It was not that they had lost out; they had never really been there at all. Now they came late and mostly from the East, entering as Yankees into the hog and hominy belt. If the Appalachian settlers were culturally limited, it led them not so much to regret their limitation as to demand that their churches conform to it. These frontiersmen had no basic aversion to doctrine, but it had to appeal to their ego and be presented movingly "in a storm." (p. 49)
(These stories are of genealogical interest to me because my maternal grandfather's Thrall and allied families were from New England by way of Ohio, who settled largely in southern Illinois in the early 1800s. Although Methodist rather than Presbyterian, they had one foot on each side of the divide.)
Later on, the book focuses more particularly on Presbyterians, their doctrine, and their role in promoting education in a state that was not very friendly to the idea at first.
Rudolph also helped abstract and index the American Home Missionary Society letters from Indiana, Indiana Letters: Abstracts of Letters from Missionaries on the Indiana Frontier to the American Home Missionary Society, 1824-1893, some of the original sources on which the book is based. So it is possible to locate and read the original (microfilmed) letters by name and/or by place. And, yes, the Allen County Public Library Genealogy Center does have the entire 385-roll collection for all the states. Has anyone indexed the letters from Illinois? or Michigan?
L. C. Rudolph, Hoosier Zion: The Presbyterians in Early Indiana (New Haven and London: Yale University Press, 1963).
L. C. Rudolph et al., Indiana Letters: Abstracts of Letters from Missionaries on the Indiana Frontier to the American Home Missionary Society, 1824-1893, 3 vols. (Ann Arbor MI: University Microfilms, 1979).
Harold Henderson, "Hoosier Zion," Midwestern Microhistory: A Genealogy Blog, posted 14 May 2012 (http://midwesternmicrohistory.blogspot.com : accessed [access date]). [Please feel free to link to the specific post if you prefer.]