I think that one of the strongest elements in this change is the different perception of God. The power of the divine that was essential to Puritan Literature. The depiction of God and how the divine controls human consciousness was of vital importance to the Puritans. The belief in Original Sin as well as the notion that human beings were doomed to live lives of sin helped to conceive of a notion of human beings as one that truly saw life as bleak and devoid of hope. This changes in the Enlightenment, when the role of the individual was vaulted above all, and where primacy was placed on the accomplishments of the individual. The role of God was still examined, and the power of the divine was still explored, yet it was done so in a context of asserting the glory of the individual. When Whitman says, “I sing of myself,” he is not kidding. He genuinely believes that the individual has a story to tell of greatness, of uniqueness, and of something that allows wonderment to be present. He is not seeing an individual chained to the condemnation of sin or acting in the eyes of a disappointed divine. Rather, there is a believe in the literature of the 19th Century American intellectual thought that suggests that individuals can do great things and can assert their identity in a manner that reflects greatness
The 18th century was the age of Enlightenment. It was the time when the 13 colonies became the United States of America, and when all ties with England and the past became severed both politically, religiously, and most importantly psychologically.
One must remember that one century before, the Puritans and other immigrant groups that came from England and all over Europe had a purpose of transferring their belief systems, and bring their past onto the new land.
However, as more diverse immigrants came in, a paradigm shift occurred in the exposure of new ideals, beliefs, and systems of faith.
Moreover, when the great philosophers such as Thomas Paine came out with mind-boggling news breakers such as Common Sense, a new era of detachment from past believes and traditions was in place.
The former literature of Winthrop, the Cotton Mahers, the Jonathan Edwards were swiftly taken to oblivion and greats such as Thomas Jefferson, Ben Franklin, the founding Fathers and many other international humanists focused on reason versus belief. It is obvious that the tendency was to move away from everything that once reminded the new Americans that they once were a group of colonists ostracized and bullied by a King. Now they were free men and, what's more, they were free to choose.
This is how the shift happened, and continues to happen as mankind continues to get to know itself.