During the Romantic period, roughly the early to mid 19th century, most of the North American continent was uncharted wilderness, the great frontier. Accordingly, writers during this time put great emphasis on the "untamed wilderness" and "rugged humanity" (see the eNotes article American Literary Criticism in the Nineteenth Century, linked below).
While British Romantics were writing in reaction to the Industrial Revolution, American Romantics were creating a national literature with the country itself at its heart. The simple beauty of nature was celebrated, and for some God could be found in nature; our task was to find that "innate power" that links us to nature (see the eNotes article "Romanticism," linked below). Emerson wrote an essay on Nature (linked below) in which he asks, "Why should not we. . .enjoy an original relationship to the universe?" Each person must find his or her own meaning through nature.
The Romantics believed in Pantheism, the idea that God existed in everything in nature and in human beings as well. They didn't believe that it was necessary to rely on ministers and books, or even to attend church to be religious. They felt one could find God everywhere in everything, especially in nature. God's existence was more evident in nature than anywhere because of the simplicity and the peacefulness found there. With the cities becoming more and more crowded, people looked for peace and quiet found outside the city, and they were able to also reconnect spiritually when they got out of the hustle and bustle of the cities.