Ben Franklin is one key Founder most likely conceded as "Deist." And indeed once in his biography he noted he identified as such. However, from the research I have uncovered, more often throughout his adult life, he thought of himself as a "Christian" -- a "rational Christian." Like Jefferson, Washington and the other key Founders, he attended Trinitarian Churches and sought communion (not in the Lord's Supper sense of the term!) with them. Though the "rational Christians" politely hoped the Trinitarians with whom they worshipped would eventually come to reject such "irrational" doctrines as original sin, the Trinity, and the infallibility of the Bible. It was the Trinitarians, especially the Calvinists, who thought such "rational Christianity" to be not Christianity at all but heresy or infidelity. Indeed, they would actively disfellow themselves from the "rational Christians" and lump them in with strict Deists like Thomas Paine. Hence the need for the key Founders to tread cautiously while dealing with the orthodox Trinitarians, speak in abstract lowest common denominator terms with them (like "Providence") and only reveal their secrets to "safe" friends, else have their public reputation damaged. And whether what the key Founders believed (a "rational Christianity" that rejected most of the fundamentals of orthodoxy) qualifies as "Christianity" is debatable.
I stress John Adams so much because his political views were so mainstream, indeed conservative for the Founding era. There is a tendency on the Christian America side to identify Franklin and Jefferson as Deists, and cast them off as outliers. But given the evidence unequivocally demonstrates J. Adams was virtually agreed with Jefferson and Franklin on their basic creed, that such a mainstream figure as J. Adams could believe in the same creed as Jefferson and Franklin demonstrates just how mainstream this creed was among the Founders. That indeed, Washington, Madison, Hamilton and many others could just as easily "fit" into Franklin's and Jefferson's religious shoes just as John Adams did.
That's the nuanced dynamic I've discovered after years of meticulously researching this issue.
But onto my contention that Franklin was not a strict Deist, but more likely thought of himself as a "rational Christian." The one time he identified as a Deist in his biography, Franklin embraced the term in a lukewarm manner:
But I was scarce fifteen, when, after doubting by turns several points as I found them disputed in the different books I read, I began to doubt of the Revelation itself. Some books against Deism fell into my hands; they were said to be the substance of the sermons which had been preached at Boyle’s Lectures. It happened that they wrought an effect on me quite contrary to what was intended by them. For the arguments of the Deists, which were quoted to be refuted, appeared to be much stronger than the refutations; in short, I soon became a thorough Deist....I began to suspect that this doctrine, though it might be true, was not very useful.
When a young man, during his defense of a young Presbyterian preacher named Samuel Hemphill accused of "heterodoxy," Franklin presented his religious creed under the auspices of "Christianity." And by the way, the heterodoxy that Hemphill preached in the American Presbyterian Church -- what the orthodox Calvinists wanted to defrock him for -- was exactly the type of "rational Christianity" that the key Founders would later embrace.
Here, in his defense of Hemphill, Franklin argues true Christianity rejects original sin.
But lest they shou’d imagine that one of their strongest Objections hinted at here, and elsewhere, is designedly overlook’d, as being unanswerable, viz. our lost and undone State by Nature, as it is commonly call’d, proceeding undoubtedly from the Imputation of old Father Adam’s first Guilt. To this I answer once for all, that I look upon this Opinion every whit as ridiculous as that of Imputed Righteousness. ’Tis a Notion invented, a Bugbear set up by Priests (whether Popish or Presbyterian I know not) to fright and scare an unthinking Populace out of their Senses, and inspire them with Terror, to answer the little selfish Ends of the Inventors and Propagators. ’Tis absurd in it self, and therefore cannot be father’d upon the Christian Religion as deliver’d in the Gospel. Moral Guilt is so personal a Thing, that it cannot possibly in the Nature of Things be transferr’d from one Man to Myriads of others, that were no way accessary to it. And to suppose a Man liable to Punishment upon account of the Guilt of another, is unreasonable; and actually to punish him for it, is unjust and cruel.
Here Franklin argues against the notion that men are justified through faith alone. He argues true Christianity elevates works over faith:
Faith is recommended as a Means of producing Morality: Our Saviour was a Teacher of Morality or Virtue, and they that were deficient and desired to be taught, ought first to believe in him as an able and faithful Teacher. Thus Faith would be a Means of producing Morality, and Morality of Salvation. But that from such Faith alone Salvation may be expected, appears to me to be neither a Christian Doctrine nor a reasonable one….Morality or Virtue is the End, Faith only a Means to obtain that End: And if the End be obtained, it is no matter by what Means.
Note how Franklin presents this as authentic Christianity and even refers to Jesus as a "Savior," but does so in the context of arguing for outright heresy. If Franklin were a Deist, why would he care about presenting his arguments as "Christian"? Further, Franklin's notion that "if the End be obtained, it is no matter by what Means" has the effect of reducing Christianity to mere morality and all good people, even professed non-Christians, to be "Christians." As John Adams put it: “I believe with Justin Martyr, that all good men are Christians, and I believe there have been, and are, good men in all nations, sincere and conscientious.”
– John Adams to Samuel Miller, July 8, 1820.
Or as George Washington put it:
To the distinguished Character of Patriot, it should be our highest Glory to add the more distinguished Character of Christian. The signal Instances of providential Goodness which we have experienced and which have now almost crowned our labours with complete Success, demand from us in a peculiar manner the warmest returns of Gratitude and Piety to the Supreme Author of all Good.
– George Washington, General Orders, Saturday, May 2, 1778
We should also understand why Franklin wouldn't fear Muslims preaching Islamic doctrines in Christian Churches. If they were good people, professed Muslims could qualify as "Christians." As Franklin put it in his autobiography:
Both house and ground were vested in trustees, expressly for the use of any preacher of any religious persuasion who might desire to say something to the people at Philadelphia; the design in building not being to accommodate any particular sect, but the inhabitants in general; so that even if the Mufti of Constantinople were to send a missionary to preach Mohammedanism to us, he would find a pulpit at his service.
In his defense of Hemphill, Franklin also notes that true Christianity views the Bible as secondary to the law of nature discovered by reason. That is, revelation's purpose is to support and complement reason, not the other way around:
Now that natural Religion, or that the Laws of our Nature oblige us to the highest Degrees of Love to God, and in consequence of this Love to our almighty Maker, to pay him all the Homage, Worship and Adoration we are capable of, and to do every thing we know he requires; and that the same Laws oblige us to the Love of Mankind, and in consequence of this Love, as well as of our Love to God, (because he requires these things of us) to do good Offices to, and promote the general Welfare and Happiness of our Fellow-creatures: That the Laws of our Nature, I say, oblige us to these things....
What Hemphill means by the first Revelation which God made to us by the Light of Nature, is the Knowledge, and our Obligations to the Practice of the Laws of Morality, which are discoverable by the Light of Nature; or by reflecting upon the human Frame, and considering it’s natural Propensities, Instincts, and Principles of Action, and the genuine Tendencies of them.
Now, that to promote the Practice of the great Laws of Morality and Virtue both with Respect to God and Man, is the main End and Design of the christian Revelation has been already prov’d from the Revelation itself. And indeed as just now hinted at, it is obvious to the Reason of every thinking Person, that, if God almighty gives a Revelation at all, it must be for this End; nor is the Truth of the christian Revelation, or of any other that ever was made, to be defended upon any other Footing. But quitting these things; if the above Observations be true, then where lies the Absurdity of Hemphill’s asserting,
That Christianity, [as to it’s most essential and necessary Parts,] is plainly Nothing else, but a second Revelation of God’s Will founded upon the first Revelation, which God made to us by the Light of Nature.
Elevating the law of nature discovered by reason over revelation opened the door for Franklin and the other key Founders to view the Bible as fallible or partially inspired. As Franklin put it in his August 21, 1784 letter to John Calder:
To which I may now add, that the[re are] several Things in the old Testament impossible to be given by divine Inspiration, such as the Approbation ascrib’d to the Angel of the Lord, of that abominably wicked and detestable Action of Jael the Wife of Heber the Kenite. If the rest of the Book were like that, I should rather suppose it given by Inspiration from another Quarter, and renounce the whole.
In that same letter Franklin demonstrates his affinity for Unitarianism: "By the way how goes on the Unitarian Church in Essex Street? and the honest Minister of it, is he comfortably supported?"
Indeed "rational Christianity" was "unitarian" not "Trinitarian." In his Sep. 28, 1772 letter to the unitarian Richard Price, Franklin expresses his interest in "rational Christianity":
Sir John has ask'd me if I knew where he could go to hear a preacher of rational Christianity. I told him I knew several of them, but did not know where their churches were in town; out of town, I mention' d yours at Newington, and offer 'd to go with him. He agreed to it, but said we should first let you know our intention. I suppose, if nothing in his profession prevents, we may come, if you please, next Sunday ; but if you sometimes preach in town, that will be most convenient to him, and I request you would by a line let me know when and where. If there are dissenting preachers of that sort at this end of the town, I wish you would recommend one to me, naming the place of his meeting. And if you please, give me a list of several, in different parts of the town, perhaps he may incline to take a round among them.
And much of what I've just outlined above is summarized in Franklin's 1790 letter to Ezra Stiles where he notes, like the "dissenters" (i.e., "rational Christians") in England he "doubts" Jesus divinity. Though he notes "Jesus of Nazareth" (not "Jesus Christ") as the world's great moral teacher. And when he lists the essentials of true religion, it's generic Providentialism with the tenets of orthodox Christianity conspicuously missing:
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.
J. Adams and Jefferson certainly and I conclude Washington, Madison, G. Morris and many other key Founders likewise could agree with that above creed. The question as to whether this system that presented itself under the auspices of "rational Christianity" actually qualifies as Christianity remains.