Unitarianism has, like most religions, changed a lot over time. When the pagan emperor, Constantine was looking for a state religion to help unify the remains of the Roman Empire, he organized the Niacene Council to encourage Christians to agree on the tenets of their faith. A major focus was on the question of the deity of Christ as encompassed in the concept of the Trinity. While all of these early Christians revered Jesus and his teachings, there was disagreement over whether he was a great prophet or in fact the son of God. The majority opted for the latter belief and those that rejected this notion of a deified Jesus were expelled from the community of Christians. These "rejects" survived in eastern Europe but did not openly adopt the Unitarian title until the 16th century in Transylvania when King Sigmound legalized the practice of Unitarianism. That didn't help when he died a few years later and his successors forced the religion back into hiding. These early practitioners were called Unitarians because of their belief that there was only one God and their rejection of the concept of the Trinity as a violation of this one-God concept.
Unitarianism was rejected as a heresy by the Catholic Church and by the early protestant churches -- hence the burning of a Unitarian philosopher in Geneva by the Calvinists. By the 18th century, in what was called the Age of Enlightenment, Unitarianism began to flourish in England and later in America, particularly in Massachusetts, where Harvard eventually evolved into what was essentially a Unitarian divinity school. At that time it was viewed as a religion of reason rather than faith since it relied on personal experience as the basis of belief. Original scriptures were seen as testimonies of personal experience. Creedal interpretations of these scriptures were rejected completely. Many also viewed the "scripture" as historic documents that had to be read with the same critical view that you might apply in reading someone's diary. From that perspective, the "Jefferson Bible" was actually a cut and paste rewrite of portions of the New Testament by Thomas Jefferson in which he eliminated what be deemed to be non-credible descriptions of miraculous acts to include only what Jefferson believed was an accurate description of "The Life and Morals of Jesus of Nazareth". Unitarians also generally rejected the notion of salvation through belief and the notion of salvation throgh predestination, favoring the notion that salvation depended on acting in a manner consistent with Christ's teachings. The religion continued its evloution through the 19th and 20th centuries. Notable Unitarians included:
Presidents John Adams
President John Quincy Adams
President Thomas Jefferson
President William Howard Taft
Presidential Candidatete Adlai Stevenson
President Millard Fillmore
Attorney General Elliott Richardson (famously fired by Nixon)
Ralph Waldo Emerson
Fannie Farmer (couldn't resist including this one)
Clara Barton (Founder of the Red Cross)
Alexander Graham Bell
Tim Berners-Lee (The real inventor of the Web)
Sir Isaac Newton
The modern Unitarian church, which merged with the Universalist Church, is rightly seen as a bastion of social liberalism. My father joined in 1941 as his form of doing penance for my grandfather's membership in the KKK. At that time, the Unitarian Church was one of the only churches taking a strong public stand on civil rights. Unitarians were also among the first to welcome gay and trans-gendered members and to permit same sex marriages.
More (and probably more accurate) information on Unitarianism is available on the website of the Unitarian-Universalist Association ay http://uua.org. Please accept that I have written this only in an effort to anse=wer a question and not in an effort to proselytize or convert anyone.
However, when historical revisionists talk about the religious beliefs of our forefathers, I encourage you to remember that many of our most important forefathers were in fact Unitarians who rejected the most fundamental tenets of what we now call Christianity (I had to say this to make my comments political).