Unitarianism derives its name from its original point of distinction with the Congregationalist churches from which its members came. Unitarians reject the notion of the Trinity, a fundamental precept of Catholic, Orthodox, and most Protestant churches, as being irrational.
In the United States, the movement took root first in Massachusetts. The first church to declare itself Unitarian was the Episcopalian King's Chapel in Boston, which revised its Book of Common Prayer in 1785 to remove all trinitarian forms. Thomas Jefferson regarded Unitarianism as the Christianity of the future.
When the Reverend Henry Ware was made the Hollis professor of divinity at Harvard, his Unitarian beliefs raised such consternation from the traditional Congregationalists that a schism resulted in the establishment of their own theological seminary in Andover in 1808.
In 1819, William Ellery Channing, the father of American Unitarianism, delivered a sermon in Baltimore that defined the movement. The American Unitarian Association was established in 1825 by the Unitarian congregational churches. Throughout Massachusetts, many communities now had both trinitarian Congregationalists as well as Unitarian churches.
In 1832, Ralph Waldo Emerson resigned from the ministry of the Second Church in Boston because of his unhappiness with administering communion. Emerson and others in the movement of transcendentalism that followed, came to prefer the term "theists" to "Christian."
---- Selected Quotes ----
Quotes regarding Unitarianism.
By William Howard Taft
I am a Unitarian. I believe in God. I do not believe in the divinity of Christ, and there are many postulates of the orthodox creed to which I cannot subscribe.
Letter to Yale University, 1899
By Abigail Adams
I acknowledge myself a unitarian Believing that the Father alone, is the supreme God, and that Jesus Christ derived his Being, and all his powers and honors from the Father. ... There is not any reasoning which can convince me, contrary to my senses, that three is one, and one three.