Unitarianism Over the Span of the American Founding

From 1750 to 1820 (ish). 

John Adams, in 1815, writes to an orthodox critic remarks that relate back to 1750.

I thank you for your favour of the 10th and the pamphlet enclosed, "American Unitarianism." I have turned over its leaves and have found nothing that was not familiarly known to me. 

In the preface Unitarianism is represented as only thirty years old in New England. I can testify as a Witness to its old age. Sixty five years ago my own minister the Reverend Samuel Bryant, Dr. Johnathan Mayhew of the west Church in Boston, the Reverend Mr. Shute of Hingham, the Reverend John Brown of Cohasset & perhaps equal to all if not above all the Reverend Mr. Gay of Hingham were Unitarians. Among the Laity how many could I name, Lawyers, Physicians, Tradesman, farmers! 

-- John Adams to Jedidiah Morse, May 15, 1815. Adams Papers (microfilm), reel 122, Library of Congress.

The reason why there was "confusion" as to how old unitarianism was relates to unitarianism being in the closet. It was not safe, in some cases not legal in say 1750, to publicly proclaim one's unitarianism.

But over time, it became safer. And I think that was probably part of the motivation behind the fervent push for liberty of conscience in some quarters (i.e., Adams' and Thomas Jefferson's, among others).

In the 1800s, Jefferson and Adams seemed downright gleeful that unitarianism was making progress. It was turning into Unitarianism, not just a theology (small u) but the official name of denominations (capital U).

As I've noted before the two options from the which to choose were Arianism and Socinianism with the former being more popular. Fast forward to 1821 and we see Jefferson in a letter to Timothy Pickering (United States Secretary of State under Presidents George Washington and John Adams) mention William Channing, Richard Price and Joseph Priestley:

I thank you for mr Channing’s discourse, which you have been so kind as to forward me. ... and read it with high satisfaction. no one sees with greater pleasure than myself the progress of reason in it’s advances towards rational Christianity. when we shall have done away the incomprehensible jargon of the Trinitarian arithmetic, that three are one, and one is three; when we shall have knocked down the artificial scaffolding, reared to mask from view the simple structure of Jesus, when, in short, we shall have unlearned every thing which has been taught since his day, and got back to the pure and simple doctrines he inculcated, we shall then be truly and worthily his disciples: ...

So off the bat Jefferson tells Pickering he read William Channings' address on "rational Christianity" which is Arian, and gave it his approval. Pickering was a fellow unitarian. Perhaps Jefferson suspects Pickering was, like what Channing argued for, an Arian.

But Jefferson was not an Arian. I would argue he was some kind of modified Socinian. But let the man speak for himself:

in the present advance of truth, which we both approve, I do not know that you and I may think alike on all points. as the Creator has made no two faces alike, so no two minds, and probably no two creeds. we well know that among Unitarians themselves there are strong shades of difference, as between Doctors Price and Priestley for example. so there may be peculiarities in your creed and in mine. they are honestly formed without doubt. I do not wish to trouble the world with mine, nor to be troubled for them. these accounts are to be settled only with him who made us; and to him we leave it, with charity for all others, of whom also he is the only rightful and competent judge. I have little doubt that the whole of our country will soon be rallied to the Unity of the Creator, and, I hope, to the pure doctrines of Jesus also.

Price was the notable influential British Arian and Priestley was his Socinian counterpart. Jefferson's comments above illustrate that the unitarianism of his time was highly individualistic. That is, it didn't exist like Trinitarianism did. Trinitarianism was institutionalized in churches with creeds; unitarianism was more of a theological philosophy that free thinking individuals attached to Trinitarian churches either flirted with or believe in.

Then it became a Church (Unitarianism with a capital U) starting at the end of the 18th Century, but more so in the 19th Century.