Many of America's Founding Fathers held views that aligned with Unitarian theology—specifically the rejection of the Trinity and the belief in the unity of God, and several definitively identified as Unitarians.
The most prominent Founding Fathers identified as Unitarians include:
John Adams: Often cited as the most "indisputably" Unitarian. Though raised Congregationalist, his home church in Quincy became Unitarian during his lifetime, and he explicitly rejected the Trinity in his private writings, calling it a "fabrication".
Thomas Jefferson: While he never officially joined a Unitarian congregation (as there were none near his Virginia home), he had strong Unitarian sympathies. He famously created the Jefferson Bible by removing miracles and references to Jesus' divinity, and he once predicted that Unitarianism would eventually become the dominant religion in America.
Benjamin Franklin: Frequently categorized alongside Jefferson and Adams as a Unitarian or a "Christian Deist". In his later life, he expressed doubts about the divinity of Jesus, though he praised his system of morals as the best the world had ever seen.
Paul Revere: He is listed by many Unitarian Universalist associations as a notable member of their historical tradition.
Abigail Adams: The wife of John Adams and an influential figure in the founding era, she shared her husband's Unitarian beliefs and is frequently included on lists of famous Unitarians.
John Quincy Adams: Though the son of John Adams, his relationship with the faith was more complex; he was a co-founder of a Unitarian church in D.C. but remained critical of some contemporary Unitarian views, at times leaning toward more traditional interpretations.
James Madison: Occasionally appears on lists as having Unitarian or Universalist leanings, though he is more commonly classified as a liberal Episcopalian.
I've attended a few Unitarian congregations and from what I can tell, their religion is basically: believe whatever you want.
