There's no smoking gun, but there's very strong evidence for it, certainly given the church he and his wife attended and the things he signed unto. And his wife stated unequivocally in writing that she was Unitarian:
President John Adams was a devout Unitarian, which was a non-trinitarian Protestant Christian denomination during the Colonial era.
He was identified as a Congregationalist by The Congregationalist Library. 1995 Information Please Almanac was cited as the source stating he was a later a Unitarian. (Source: Ian Dorion, “Table of the Religious Affiliations of American Founders”, 1997).
From: Peter Roberts, “John Adams” page in “God and Country” section of “Science Resources on the Net” website (http://www.geocities.com/peterroberts.geo/Relig-Politics/JohnAdams.html; viewed 23 November 2005):
Adams was raised a Congregationalist, but ultimately rejected many fundamental doctrines of conventional Christianity, such as the Trinity and the divinity of Jesus, becoming a Unitarian. In his youth, Adams’ father urged him to become a minister, but Adams refused, considering the practice of law to be a more noble calling. Although he once referred to himself as a “church going animal,” Adams’ view of religion overall was rather ambivalent.
John Adams, the second U.S. President rejected the Trinity and became a Unitarian. It was during Adams’ presidency that the Senate ratified the Treaty of Peace and Friendship with Tripoli, which states in Article XI that:
As the government of the United States of America is not in any sense founded on the Christian Religion - as it has in itself no character of enmity against the laws, religion or tranquility of Musselmen, - and as the said States never have entered into any war or act of hostility against any Mehomitan nation, it is declared by the parties that no pretext arrising from religious opinions shall ever produce an interruption of the harmony existing between the two countries. (Charles I. Bevans, ed. Treaties and Other International Agreements of the United States of America 1776-1949. Vol. 11: Philippines-United Arab Republic. Washington D.C.: Department of State Publications, 1974, p. 1072).
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.”
It’s always interested me that the New England branch of Calvinism so quickly morphed into Unitarianism and Transcendentalism (basically Eastern mysticism), while the southern branch kept more true to its roots. Maybe it’s because the New Englanders were able to set up a theocracy, while the Scotch-Irish remained outsiders. It’s been said that the more the church is persecuted, the purer it is. Establishment—the unification of religion with the secular state—is ruinous to faith.