William Cullen Bryant is best known for writing calm, thoughtful poems about the natural world. He experimented with a bunch of different kinds of forms and meters, but he showed a pretty consistent interest in streams, flowers, birds, and all that stuff.
Bryant also often used these poems about nature to explore philosophical and spiritual themes. Some of his most famous poems, like "To a Waterfowl" and "To the Fringed Gentian," talk about the same kind of positive spiritual feelings that we see at the end of "Thanatopsis." He stays away from talking about religion directly, but he does hint that there’s something in the universe that’s looking out for us.