As American Christians moved to the frontier away from the established communities along the eastern seaboard, they also moved away from their Puritan and Calvinistic assessment of human nature. If we could conquer the west, build cities where there had been only wilderness, and if this was the fruit of our manifest destiny and our democratic ideal, then the "terrible honesty" of Calvinistic convictions, to use Ann Douglas' phrase, made little sense. In this context, Americans are rugged, capable and basically good people. And so, Pelagius became our patron saint and Charles Finney his main spokesman.
It is no accident that most of the Pelagianizing movements just mentioned, sprang up on the American frontier in a region in upper state New York, known to historians as the "burned over district," a region which produced millennialism and Millerites, Joseph Smith and Mormonism, Alexander Campbell and the Restoration movement, the Shakers and a host of others, all which grew up in the Wake of Charles Finney and his new measures. From a Reformed perspective, the "burned over district" is a kind of a theological Bermuda Triangle.
(h/t Reformed Theology via Craig)
Culture, geography and religion are probably more tightly connected then we imagine, check out these very interesting religious maps of America.