Is there any official conversion required to become Unitarian? According to something “NowHere This” said previously, it is my impression one can call oneself Jewish, Christian, atheist, and so on, while being Unitarian at the same time. In the case of Judaism, there are Jews who also consider themselves Buddhist, Quaker, Christian Scientist, Hebrew Catholic, or Messianic, while retaining their Jewish religious identity. This may be challenging, however, depending on which movement of Judaism one originally belongs to. I would imagine for mainline Protestants it would be VERY challenging since Unitarianism does not believe in the Trinitarian G-d.
That’s my understanding, as well – that one can be a Buddhist-Unitarian, a Quaker-Unitarian, a Jewish-Unitarian, even an atheist-Unitarian. Its
sine qua non is a commitment to love and service of neighbor, both on a person-to-person level and on the level of promoting social justice.
Historically, as I understand it, the Unitarians and the Deists were fairly close in orientation during the 18th century (Thomas Jefferson, for example, was both a Unitarian and a Deist). Nonetheless, Unitarians of the 18th and 19th centuries remained a Christian denomination.
When Ralph Waldo Emerson came along in the 19th century, he resigned his post as Unitarian minister, because he felt he could no longer – in good conscience – administer the Lord’s Supper; nor, frankly, did he consider himself a Christian (even though he was very much a believer in God and in the moral law). He was, in fact, the spiritual father of the New England Transcendentalists.
He tried to get the Unitarian church to follow him in moving even further afield from Christianity; it refused to do so, and he respectfully left the church. The rest is history – the New England Transcendentalists were a non-Christian offshoot of Unitarianism, which embraced mystical communion with nature and were also influenced by Eastern religions, particularly Hinduism.
Come the late 19th and early 20th century, and Unitarians “reabsorbed”, if you will, the Transcendentalist orientation; so, in the end, Emerson got his way

Frank Lloyd Wright is a good example of this merging of the Unitarian and the Transcendentalist vision (he loved Emerson’s writings, yet was a member of a Unitarian congregation; he even designed a famous Unitarian church in Oak Park Illinois, called “Unity Temple”). Thus, at a Unitarian sermon today, in the early 21st century, you
may hear a gospel passage being discussed, but you might also hear a reading from Whitman’s Leaves of Grass, or perhaps Thoreau’s Civil Disobedience; or Emerson’s essay on “Nature”; or, perhaps, a Buddhist parable.