Kristo Miettinen has a post at American Creation on John Adams' personal heterodox religion and his promotion of Christianity in the Massachusetts state constitution of 1780 he helped to pen. The relevant language in the state constitution is:
As the happiness of a people, and the good order and preservation of civil government, essentially depend upon piety, religion and morality; and as these cannot be generally diffused through a community, but by the institution of the public worship of God, and of public instructions in piety, religion and morality: Therefore, to promote their happiness and to secure the good order and preservation of their government, the people of this commonwealth have a right to invest their legislature with power to authorize and require, and the legislature shall, from time to time, authorize and require, the several towns, parishes, precincts, and other bodies politic, or religious societies, to make suitable provision, at their own expense, for the institution of the public worship of God, and for the support and maintenance of public Protestant teachers of piety, religion and morality, in all cases where such provision shall not be made voluntarily.
And the people of this commonwealth have also a right to, and do, invest their legislature with authority to enjoin upon all the subjects an attendance upon the instructions of the public teachers aforesaid, at stated times and seasons, if there be any on whose instructions they can conscientiously and conveniently attend.
Provided, notwithstanding, that the several towns, parishes, precincts, and other bodies politic, or religious societies, shall, at all times, have the exclusive right of electing their public teachers, and of contracting with them for their support and maintenance.
And all moneys paid by the subject to the support of public worship, and of the public teachers aforesaid, shall, if he require it, be uniformly applied to the support of the public teacher or teachers of his own religious sect or denomination, provided there be any on whose instructions he attends; otherwise it may be paid towards the support of the teacher or teachers of the parish or precinct in which the said moneys are raised.
Any every denomination of Christians, demeaning themselves peaceably, and as good subjects of the commonwealth, shall be equally under the protection of the law: and no subordination of any one sect or denomination to another shall ever be established by law.
The conundrum is how could someone with with views as heterodox as Adams support such a thing? Miettinen writes:
Adams knew that virtue sufficient for civil society was found in men of many faiths, and he arguably even would have called all such men "Christian" regardless of faith, e.g. a pious Muslim who was a model of civic virtue might be pleasing to God, and saved, and therefore living the life Christ intended (thereby qualifying as Christian), despite never hearing of, or believing in, Christ.
This is not nearly the same thing as believing that a society that inculcates Islam in its citizens has the same prospects of success and virtue as a neighboring society that inculcates Christianity. One religion is better than the rest, at least on Adams' view, and that was the religion intended by Adams for America. It was a broad vision of Christianity that embraced Unitarianism (then based at Harvard and surging in Massachusetts) but probably not Universalism (or Catholicism).
I agree that Roman Catholicism was not Adams' cup of tea; Adams was arguably an anti-Catholic bigot. From a letter to Jefferson, May 1816:
"I do not like the reappearance of the Jesuits.... Shall we not have regular swarms of them here, in as many disguises as only a king of the gipsies can assume, dressed as printers, publishers, writers and schoolmasters? If ever there was a body of men who merited damnation on earth and in Hell, it is this society of Loyola's. Nevertheless, we are compelled by our system of religious toleration to offer them an asylum."
Adams also remarked to Jefferson in 1821: "Can a free government possibly exist with the Roman Catholic religion?"
However, Adams himself was a theological universalist in the sense of denying eternal damnation:
"I believe too in a future state of rewards and punishments too; but not eternal."
-- To Francis van der Kemp, July 13, 1815.
More importantly George Washington gave such theological universalism his imprimatur, which I think settles the matter that such was part of the civil religion. As he wrote to the Universalist Church in Philadelphia:
I thank you cordially for the congratulations, which you offer on my appointment to the office I have the honor to hold in the government of the United States.
It gives me the most sensible pleasure to find, that, in our nation, however different are the sentiments of citizens on religious doctrines, they generally concur in one thing; for their political professions and practices are almost universally friendly to the order and happiness of our civil institutions. I am also happy in finding this disposition particularly evinced by your society. It is, moreover, my earnest desire, that all the members of every association or community, throughout the United States, may make such use of the auspicious years of peace, liberty, and free inquiry, with which they are now favored, as they shall hereafter find occasion to rejoice for having done.
With great satisfaction I embrace this opportunity to express my acknowledgments for the interest my affectionate fellow-citizens have taken in my recovery from a late dangerous indisposition; and I assure you, Gentlemen, that, in mentioning my obligations for the effusions of your benevolent wishes in my behalf, I feel animated with new zeal, that my conduct may ever be worthy of your favorable opinion, as well as such as shall, in every respect, best comport with the character of an intelligent and accountable being.
When John Adams in his personal letters made odd comments equating Christianity with Zeus worship or Hinduism, I think he was out of the mainstream. However, the idea that the "end" of religion was to make men moral, and as such, if the "end" was met, the means didn't matter -- hence all good people were "Christian" -- was quite mainstream among the key Founders.
"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.”
– “Dialogue between Two Presbyterians,” April 10, 1735.
“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.”
– To Samuel Miller, July 8, 1820.
"My fundamental principle would be the reverse of Calvin's, that we are to be saved by our good works which are within our power, and not by our faith which is not within our power."
-- To Thomas B. Parker, May 15, 1819.
"[F]or no man, who is profligate in his morals, or a bad member of the civil community, can possibly be a true Christian, or a credit to his own religious society.
"I desire you to accept my acknowledgments for your laudable endeavours to render men sober, honest, and good Citizens, and the obedient subjects of a lawful government."
-- George Washington, Letter to General Assembly of Presbyterian Churches, May 1789.
So subtract from civil Christianity justification through faith.
And for the sake of space I won't detail the key Founders' view on the Bible. I'll just assert it cannot be gleaned that they believed the Bible infallible. I have quotations from Jefferson, J. Adams, and Franklin rejecting such. And Washington and Madison never publicly held up the Bible an inerrant, infallible book. At best, a consensus dictates they believed the Bible at least partially inspired, many believing it more, some believing it less.
So we end up with a civil Christianity that lacks belief in 1) the Trinity and related orthodox doctrines, 2) eternal damnation, 3) justification through faith, and 4) the infallibility of the Bible.
And because virtue is central to this public creed, if it can be found in other religions, they too have a place at the table of "publick religion."
Is there a problem here? I would say yes. The same political theological problem exists today as it did during the Founding era. Lot's of things have changed since then, but this, I assert, is not one of them: The "orthodox" do not consider what's outlined in those four points to be "real Christianity." Rather, they are poison pills.
The way the Founders initially threaded this needle was leaving religion to the states. One irony of ironies is that Jefferson and Madison who handled state religion policy for Virginia that had the strongest degree of separation of Church & State and J. Adams who spearheaded Massachusetts' model that had the strongest degree of integration between Church & State possessed a personal religious creed that was agreed on the basics (i.e., a "rational Christianity" that denied the Trinity, Incarnation, Atonement, eternal damnation, and infallibility of the Bible).
And it's important to note that John Adams and many other New England Congregationalists thought this "rational Christianity" to be every bit of deserving public funds. But the orthodox did not. And that's because to them this "rational Christianity" was NOT Christianity, but, to use a present analogy, like Mormonism, a creed that calls itself Christianity but is not. When the orthodox scholar Gregg Frazer terms this creed "theistic rationalism" he is being true to this historic dynamic as well as the personal religious convictions of most orthodox Trinitarian Christians through out history.
The history of disestablishment in New England supports my analysis. In the mid to late 18th Century theological unitarians abounded as both ministers and laymen in the Congregational Church. Those very same churches also had orthodox Calvinists and the unitarians threaded the needle by just ignoring the divisive Trinitarian doctrines about which they differed. But slowly unitarianism came out of the closet. And when it did the orthodox Calvinists who did NOT think Unitarianism to be "Christianity" actively disfellowed themselves from the Unitarians and sought control over church property and establishment aid. They lost a series of decisions at the state level in Mass. (this was not a federal issue as of yet). Most notably was the Dedham decision which left much Church property and establishment aid in the hands of Unitarians.
As I noted above, this was, to the orthodox, a poison pill. That what they regarded as fake Christian heretics were getting government money under the auspices of being a "Protestant Christian" sect was too much for them. This was the impetus that led Massachusetts to finally disestablish in 1833.
Madison, of course, was well aware of these potential spats and that's what drove his Memorial and Remonstrance. Under the Virginia model, Christianity would get no public aid. As Jefferson's Statute (which Madison helped to see pass) famously notes:
that to compel a man to furnish contributions of money for the propagation of opinions which he disbelieves, is sinful and tyrannical; that even the forcing him to support this or that teacher of his own religious persuasion, is depriving him of the comfortable liberty of giving his contributions to the particular pastor, whose morals he would make his pattern,...
II. Be it enacted by the General Assembly, That no man shall be compelled to frequent or support any religious worship, place, or ministry whatsoever, nor shall be enforced restrained, molested, or burthened in his body or goods, nor shall otherwise suffer on account of his religious opinions or belief; but that all men shall be free to profess, and by argument to maintain, their opinion in matters of religion, and that the same shall in no wise diminish, enlarge, or affect their civil capacities.
Madison's Memorial and Remonstrance argues for a "no cognizance" by government of religion standard. Madison, in his notes on the Mem. & Remon. informs in more detail why government has no right to take cognizance of religion. It has to do with this very issue that we are currently debating: What is Christianity?
In Roman numeral V of his notes, Madison ponders the question “What is Xnty?” This is relevant because in Patrick Henry's bill against which Madison remonstrated only “Christianity” and not other religions were eligible for aid. In V6 he discusses that some view the entire Bible as divinely inspired, some view only “essential parts” as divinely inspired; in V7 he notes some believe if a creed rejects certain key doctrines it is not Christian even if it calls itself Christian; in V8 he notes Trinitarianism, Arianism, Socinianism, (the latter two are forms of theological unitarianism) asking which of them would qualify as “Christian” under the bill; and in II6 he notes the case of “primitive Christianity,” “Reformation” and “Dissenters formerly.” He concludes that unless Christianity is specifically defined in the bill, judges might have to answer what is heterodoxy v. what is orthodoxy. And that in turn will “dishonor Christianity.”
So Madison believed that government itself had no natural right to decide what is Christianity? This is essential: If government is going to support Christianity and not other religions, it has to define Christianity, something by right, it could not do. And indeed, this is EXACTLY what ended up happening in Massachusetts. That state Supreme Court which had a number of Unitarians on the bench had to decide whether theological unitarianism qualified as "Christianity," and they held yes it did. To many orthodox this was akin to government declaring declaring a dog's tail is a fifth leg.
Of course, this problem is mostly obviated by government granting rights to "religion" in general as opposed to Christianity in particular. And this is exactly what was done in the US Constitution.