Book on Unitarianism:

If so interested, check out this book, Unitarianism in America, by George Willis Cooke, the entire thing available free, online. It shows how, as an ecclesiastical matter, Unitarianism really took off in New England only. Most New England churches which preached theological unitarianism, some for over fifty years, didn't change their creeds until around the 19th Century. This has led many to mistake unitarianism as a 19th century phenomenon. While ecclesiastical Unitarianism may have been, theological unitarianism had been believed in by dissenters all over the nation from the mid-18th century onward.

One reason why theological unitarianism may have appealed to so many of our key Whig founders is that so many of their Whig counterparts from England were likewise theological unitarians. Indeed, many of the big "Christian" names from England were actually Arian heretics -- for instance Milton, Newton, and Locke. Some lesser well known but still very influential English rationalist Whigs, also Arians, who influenced our founders include Samuel Clarke, and Shaftesbury. Of course, many British unitarian Whigs were Socinians, most notably Joseph Priestly, whose religious thought probably more than any other British Divine influenced our key Founders.

One reason why these unitarians were "dissenters" in England was because it didn't become legal there to deny the Trinity publicly until 1813. Doing so, in some Christian Commonwealths, for instance Calvin's Geneva, could get one burned at the stake, see Servetus.