Franklin is one that both the secular left and religious right likely concede as "Deist." Though some on the religious right accurately note Franklin's speech at the Constitutional Convention where he professes theism.
Still, the religious right tends towards the narrative that Franklin and Jefferson were somewhat "Deists," the rest Christian. Franklin actually gives fewer smoking gun quotations of a belief in militant unitarianism than does John Adams. If "right" = Christian and "left" = Deism, Adams was arguably (at least in his personal theology) to the left of Franklin.
Franklin never for instance bitterly and militantly mocked the Trinity and Incarnation as Adams did. Franklin did "doubt" Jesus' divinity in his letter to Ezra Stiles which could be read as a polite way of denying Jesus' divinity to an orthodox gentlemen.
This is the passage in context:
[B]ut I apprehend it has received various corrupting Changes, and I have with most of the present Dissenters in England, some Doubts as to his Divinity.
What I put in bold arguably supports the not just "doubt" but "deny" thesis. Those "dissenters" in England were self described unitarians who denied Jesus' divinity. And "corrupting changes" no doubt refers to Unitarian Joseph Priestley's "Corruptions of Christianity" text which denies Original Sin, Trinity, Incarnation, Atonement, and plenary inspiration of the Bible as false "corruptions" of Christianity.
I'm looking for evidence of Franklin calling himself a unitarian. In this letter to unitarian of the Arian bent Richard Price, Franklin intimates that they both believe in the same "rational Christianity."
And I found this interesting link, Sermon: “Dear Mr. Isaacson, Ben Franklin was Unitarian,” A Sermon by Charles Blustein Ortman, September 19, 2003.
Much of what it reproduces I already know and have blogged about. The sermon tries to connect Franklin to the unitarian label. This is new information about which I hadn't known:
These associations [with unitarian friends] are hardly coincidental. Still they don’t meet the test of placing Benjamin Franklin within a Unitarian congregation. That information can be found though, in the records of Essex Street Chapel in London. Earl Morse Wilbur, renowned Unitarian historian, reports that not only was Franklin in attendance at the first Unitarian service held in England on April 17, 1774, but that he had a long standing friendship with the minister there, Theopholis Lindsey, and that, “he continued to worship here as long as he remained in England.” (Wilbur)
Update: Let's not forget Franklin's classic letter to John Calder where he asserts "the[re are] several Things in the old Testament impossible to be given by divine Inspiration,..."
There he also writes: "By the way how goes on the Unitarian Church in Essex Street? and the honest Minister of it, is he comfortably supported?" I think it's safe to conclude Franklin was self consciously "unitarian."
Update II: The "honest minister" is none other than the Rev. Lindsey.