He quotes Barton:
We have the same thing when you look at Quakers. You see Quakers were founded by William Penn in Pennsylvania. I’ll lay you odds there’s no chance that William Penn would be a Quaker today, even in the denomination he founded, he would not be a part of. We look at it the way it is today and say it must have been the way they were back then.
And the example of that is what happens when you look at Universalist Unitarians; certainly not a denomination that conforms to biblical truth in any way but as it turns out, we have a number of Founding Fathers who were Unitarians. So we say, oh wait, there’s no way the Founding Fathers could have been Christians; they were Unitarians. Well, unless you know what a Unitarian was in 1784 and what happened to Unitarians in 1819 and 1838 and unless you recognize they used to be a very evangelical Christian denomination, we look at what they are today and say the Founding Fathers were Unitarians, and say, there’s no way they were Christians. That’s modernism; that’s not accurate; that’s not true.
As Throckmorton notes, Penn didn't found Quakerism, George Fox did.
This is a comment I left there:
The idea that Unitarians (of the Founding era) can’t be Christians is not modernism; it’s orthodoxy.
Founding era Unitarianism (I term it “unitarianism” and leave the “u” uncapitalized because the unitarianism of the Founding era — mid to late 18th Century — usually wasn’t an official denomination, but a theology) defined itself by disbelieving in the Trinity. They identified and understood themselves to be “Christians.” It was the orthodox of that era and of today who claim, no, you must believe in the Trinity to be a “mere Christian” (as CS Lewis would term it).
I think the kernel of Truth in Barton’s claim might be the unitarians of the Founding era weren’t quite like today’s UUs. They were quite theistic, devout and very often biblical.
In this sense, they may be more like today’s Mormons or Jehovah’s Witnesses. Non-Trinitarians, but nonetheless very devout and theistic. Though the Founding era unitarians were also very rationalistic, “enlightened” and “liberal” for their day.
Though I am starting to see some consistency in Barton’s understanding of Christian minimums. If non-Trinitarians like Mormons can qualify as “Christians,” so too can many if not most of the Founding era unitarian Christians.