Overall, this is fairly well written and reasoned out, althought the author falls prey to America’s own little creation myth in citing so much Puritan influence, when in fact the government of Viriginia was well established over a decade prior. These were no Puritans. They were Anglican.
Second, the author either fails to notice or deliberately glosses over another key influencing trend during the 1870 - 1930 era cited as being the source of a putative American “falling away” from Christian faith of the Protestant variety. One needn’t look too far or too hard to understand that this corresponds almost precisely with the waves of ... Catholic immigration.
It also coincides with the explosion of overweening, intrusive government made possible by the Federal leviathan having run roughshod over the Constitution during the so-called Civil War, and most especially doing so in the Reconstruction era. These things did not occur in a vacuum, and occurred to the dismay and frequent resistance of the descendants of Old Virginia and the south in general, among whom the Christian faith remains much stronger than in the population as a whole.
The Puritans were Anglicans trying to purify the church of England, which is why they were called “Puritans”. The Pilgrims were separatist (independent of the C of E) congregationalists. In time, the Pilgrims led the New England Puritans to become “Congrgationalists.” But, originally, the Puritans were Anglican. BTW, at the time the Anglicans had a reformed confession of faith. Consequently, most of the difference between Episcopalians/Anglicans and Congregationalists was in the area of ecclesiology.
Overall, the reformed Christians in New England - many of them Puritans - had more influence culturally in the formation of the country than Anglicans. The lines of influence were somewhat blurred, however, because they shared so much theology.