Abigail Adams was a devout Unitarian. The Unitarian Church was a non-trinitarian Protestant Christian denomination during the Colonial era.
Abigail Adams (born Abigail Smith) is regarded as one of the most influential women in American history. Abigail Smith Adams was the wife of John Adams, the second U.S. President, and she did much to shape the office of the First Lady of the United States forever after.
From: B. J. Lossing, Signers of the Declaration of Independence, George F. Cooledge & Brother: New York (1848) [reprinted in Lives of the Signers of the Declaration of Independence, WallBuilder Press: Aledo, Texas (1995)], pages 2:
In 1766 Mr. Adams [John Adams] married Abigail Smith, the amiable daughter of a pious clergyman [a liberal Congregationalist] of Braintree, and soon afterward he removed to Boston.
From: Laurie Carter Noble, "Abigail Adams" article on Unitarian Universalist Historical Society website (http://www.uua.org/uuhs/duub/articles/abigailadams.html; viewed 13 November 2005):
Born in the parsonage of the North Parish Congregational Church of Weymouth to the Rev. William Smith and Elizabeth Quincy, Abigail was raised simply and without pretension, though her relatives, especially on her mother's side, were among the leading families of their time...
Her father, William Smith (1707-1783), was a liberal Congregationalist, who often exchanged pulpits with his friend, Ebenezer Gay. Smith was an Arminian. He did not preach the doctrines of predestination, original sin, or the full divinity of Christ. Rather, he emphasized the importance of reason and morality in religious life. This simple faith his daughter Abigail confessed when she was received into membership in the Weymouth church on June 24, 1759.
That same year, Abigail Smith met John Adams. By 1762 they were exchanging frankly affectionate love letters full of mischievous humor. Their wedding, on October 25, 1764, began one of history's great partnerships. They were lovers, friends, counselors, and mentors to one another into old age...
John and Abigail Adams were active members of the First Parish Church in Quincy, which was already unitarian in doctrine by 1753. Although she did not sign the membership book (John did), she attended the church, supported it, and showed active concern and care for its ministry. She is a celebrated figure in her congregation's tradition. Abigail's theology is clearly stated in her correspondence. Writing to her son, John Quincy Adams, on May 5, 1816, she said, "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." On January 3, 1818, writing to her daughter-in-law, Louisa, Abigail wondered "when will Mankind be convinced that true Religion is from the Heart, between Man and his creator, and not the imposition of Man or creeds and tests?" Like many early Unitarians she discounted sectarian claims and was "assured that those who fear God and work righteousness shall be accepted of him, and that I presume of what ever sect or persuasion."
Early in October, 1818, Abigail fell ill with typhus and died several weeks later. She was buried in the cemetery of First Church in Quincy. John Adams died in 1826 during the presidency of John Quincy Adams.