The show does a good job showing how English Puritan Protestantism greatly impacted America. The Puritans focused on the Old Testament, unique among Christians at the time, and saw themselves as the remnant of the true faith leaving Egypt (England) for the promised land (America). Seeing America as a blank slate and a "city upon the hill" infused the United States with the belief that religion is critical for the country and America is God's country.
That belief has greatly impacted the country: Dissenter religions like Methodism which could claim direct contact with God flourished, Americanist religions like Mormonism were born, Manifest Destiny gave the country a conquering spirit over the American Indian nations, and politics and religion have been closely intertwined in issues ranging from education to civil rights to geopolitics.
God In America is long but it is an intimate, person-driven account of how religion has impacted the United States.