William Ellery Channing was the foremost Unitarian preacher in the United States in the early nineteenth century and, along with Andrews Norton (1786–1853), one of Unitarianism's leading theologians. Channing was known for his articulate and impassioned sermons and public speeches, and as a prominent thinker in the liberal theology of the day. His religion and thought were among the chief influences on the New England Transcendentalists although he never countenanced their views, which he saw as extreme. His espousal of the developing philosophy and theology of Unitarianism was displayed especially in his "Baltimore Sermon" of May 5, 1819, given at the ordination of the theologian and educator Jared Sparks (1789–1866) as the first minister of the newly organized First Independent Church of Baltimore.

Unitarianism is a nontrinitarian branch of Christianity. Unitarian Christians affirm the unitary nature of God as the singular and unique creator of the universe, believe that Jesus Christ was inspired by God in his moral teachings and that he is the savior of humankind, but he is not equal to God himself.