It may surprise you to know that John Milton was not a Christian, but a heretic. That is, if one defines "Christianity" as Trinitarian and accepting of the Nicene Creed. I've been doing some research on Milton, and the consensus seems to be that Milton (like Isaac Newton and many others) was an Arian, which is a form of theological unitarianism which views Jesus as a divine but created being, inferior to God the father.
America was born of theological dissent. As I noted in my past post, the dissident Whig tradition in England arguably most greatly impacted our founders in the ideological sense. And they were disproportionately made up of theological unitarians which was the ultimate form of theological dissent in a nation, England, that officially established Trinitarian Christianity. Indeed, they had a law which made it a crime to deny publicly the Trinity, which was finally removed the books in 1813, the subject of this letter by John Adams to Thomas Jefferson, two fervent unitarians, where Adams goes so far to say that had God Himself revealed the doctrine of the Trinity to Adams on Mt. Sinai with Moses, he still wouldn't believe it because man's reason proved that one was not three period.
Ben Franklin in expressing his unitariasm also testifies that many of those Whig dissident thinkers were unitarians. Franklin writes to Ezra Stiles, as President of Yale, a very prominent orthodox Christian (Franklin's tone is very polite; he practically walks on egg shells):
You desire to know something of my Religion. It is the first time I have been questioned upon it: But I do not take your Curiosity amiss, and shall endeavour in a few Words to gratify it. Here is my Creed: I believe in one God, Creator of the Universe. That He governs it by his Providence. That he ought to be worshipped. That the most acceptable Service we can render to him, is doing Good to his other Children. That the Soul of Man is immortal, and will be treated with Justice in another Life respecting its Conduct in this. These I take to be the fundamental Principles of all sound Religion, and I regard them as you do, in whatever Sect I meet with them. As to Jesus of Nazareth, my Opinion of whom you particularly desire, I think the System of Morals and his Religion as he left them to us, the best the World ever saw, or is likely to see; but 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: tho' it is a Question I do not dogmatise upon, having never studied it, and think it needless to busy myself with it now, when I expect soon an Opportunity of knowing the Truth with less Trouble. I see no harm however in its being believed, if that Belief has the good Consequence as probably it has, of making his Doctrines more respected and better observed, especially as I do not perceive that the Supreme takes it amiss, by distinguishing the Believers, in his Government of the World, with any particular Marks of his Displeasure.
So Franklin says that like "most of the present Dissenters in England" he doubts Jesus' divinity. Those dissenters were the English Whigs, in whose writings the key Founders were imbibed. Franklin and the other key Founders carried on correspondence with many of the dissenters alive during their era, including Joseph Priestly, Richard Price, and James Burgh. In fact, Franklin, Priestly, Price and Burgh were part of a club called the "Honest Whigs."
Likewise, when Franklin writes that Jesus' religion "has received various corrupting Changes," that phrase has a specific meaning. Fellow Whig Joseph Priestly coined the phrase "the corruptions of Christianity," and they included "a trinity of persons in the godhead, original sin, arbitrary predestination, atonement for the sins of men by the death of Christ, and ... the doctrine of the plenary inspiration of the scriptures." In other words, Franklin like Priestly rejected all of the doctrines of orthodox Christianity.
Finally, let's read again the tenets of Franklin's creed. My research shows that this is exactly what Jefferson, Washington, Adams, and Madison also believed:
I believe in one God, Creator of the Universe. That He governs it by his Providence. That he ought to be worshipped. That the most acceptable Service we can render to him, is doing Good to his other Children. That the Soul of Man is immortal, and will be treated with Justice in another Life respecting its Conduct in this.
These are, according to Franklin (and the others), "the fundamental Principles of all sound Religion." And by this they included, not just the Christian sects, but "all sound Religion." The world religions with which they were familiar included, at the very least, Christianity (both Protestant and Catholic), Judaism, Deism, Unitarianism, Islam, Hinduism, and Native American spirituality. Also Pagan Greco-Romanism qualified as "sound religion." So when the Founders believed all these world religions contained the same Truth as Christianity and were thus valid ways to God, it is the above passage which is the "Truth" of Christianity they thought valid. What comes from the creeds -- the Trinity, the Incarnation, the Atonement, the plenary inspiration of the scriptures, etc. -- in other words what distinguishes Christianity from the other world religions, was not "Truth" but "corruption."
The Trinity is only bearable according to Franklin (but not Adams and Jefferson who thought it to be absolutely pernicious) because Christians nonetheless believed in those tenets which all sound religions believe. And that, to Franklin, was what really mattered.
Finally, in his letter to Stiles Franklin asks in his PS:
(I confide, that you will not expose me to Criticism and Censure by publishing any part of this Communication to you. I have ever let others enjoy their religious Sentiments, without reflecting on them for those that appeared to me insupportable and even absurd. All Sects here, and we have a great Variety, have experienced my Good will in assisting them with Subscriptions for building their new Places of Worship, and as I have never opposed any of their Doctrines I hope to go out of the World in Peace with them all.)
Because of the legal and social entrenchment that orthodox Christianity had not just in England but also in the colonies and then the newly formed United States (at the state level), one could still have one's reputation ruined if one wore one's infidelity on one's sleeve (as Thomas Paine did). Therefore prudent public figures kept religious secrets. Madison and Washington did not reveal as many of the specifics of their beliefs as Franklin, Adams, and Jefferson did. But they did hold religious secrets. And the evidence otherwise demonstrates that Washington and Madison believed in these same unitarian principles which Dr. Gregg Frazer has termed "theistic rationalism."