In his excellent book Founding Faith, Steven Waldman notes America was founded not as a "Secular" (the myth of the left) or a "Christian" (the myth of the right) Nation, but a religiously free nation. I think you could add to that a religiously "equal" nation as well. Dr. Gregg Frazer has noted that America was founded to be "religious" not "Christian" and America's key Founders were themselves "religious" but not "Christian" men. And I think that component needs to be understood in the mix as well.
Here I argue that the notion of a religiously free and equal nation is arguably incompatible with the idea of a "Christian Nation." There is a false quotation, widely spread on the Internet attributed to Patrick Henry:
It cannot be emphasized too strongly or too often that this great Nation was founded not by religionists, but by Christians; not on religions, but on the Gospel of Jesus Christ. For that reason alone, people of other faiths have been afforded freedom of worship here.
It's this type of sentiment that expositors of the Christian Nation thesis need to rely on to reconcile "Christian Nation" with religious liberty. That's why you see it so widely quoted. But as noted, the quote is a myth and the idea of religious liberty (and equality) applied beyond "the Christian sects" is arguably irreconcilable with the Christian Nation thesis. That's one reason why some (not all) Christian Nationalists argue the religious clauses were meant to apply to "Christianity only." Were that true one could still make the "Christian Nation" claim. And no doubt some/many in the population understood the clauses that way. But not the Founding Presidents. And they wrote "religion" not "Christianity" into the text of the Constitution. So when, for instance, George Washington, as President wrote to a Jewish Congregation at Newport and noted --
"It is now no more that toleration is spoken of, as if it was by the indulgence of one class of people, that another enjoyed the exercise of their inherent natural rights. For happily the government of the United States, which gives to bigotry no sanction, to persecution no assistance, requires only that they who live under its protection should demean themselves as good citizens, in giving it on all occasions their effectual support.…May the children of the Stock of Abraham, who dwell in this land, continue to merit and enjoy the good will of the other inhabitants, while every one shall sit in safety under his own vine and fig-tree, and there shall be none to make him afraid."
-- it seems impossible to claim with a straight face that the religion clauses meant "Christianity" only. Likewise if the Free Exercise Clause applied beyond "Christianity" the Establishment Clause, in principle, by logical necessity did as well. Let's look at the text of the First Amendment:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof;...
The text uses the term "religion" once. The "thereof" in the Free Exercise Clause relates back to term "religion" in the Establishment Clause. As Philip Hamburger noted it is logically impossible for that one term used in one place to mean two different things. The EC and FEC are like Siamese twins who share the same heart. If it's a "religion" for FEC purposes, it is also a "religion" for EC purposes.
With that said, let me address the issue of religious pluralism. The population of America during the Founding era was roughly 98% Protestant Christian in a formal/nominal sense (with men like Jefferson who rejected every single tenet of Christian orthodoxy meeting this minimal definition of "Protestant Christian"). The largest non-Christian population were probably the Native Americans (I think I can safely say they outnumbered Jews). There weren't that many Roman Catholics outside of Maryland. I doubt the Jews broke one thousand in number (I could be wrong). And though Muslims were not non-existent in America, you probably could them on two hands if not one. I know of no Hindus or Buddhists (perhaps they did exist; I'm just not aware of them).
Expounding on this dynamic two of my American Creation co-bloggers Tom Van Dyke and Kristo Miettinen wrote what follows. First TVD:
I object to the current We Are the World sentiment that Muslims and Hindus [or atheists, but that's a different discussion] have any connection with the Founding principles. For the simple reason that there weren't any around.
Although some Founders mention those religions in the abstract, there's no evidence they had any genuine understanding of them.
Then Kristo reacting to my assertion that "[t]here was no alliance of Jews & Christians during the Founding era. It was 'Protestant Christians' in one box - the 'in' box - and Jews, Roman Catholics, Muslims and pagans in the other more 'out' box."
I'd say (but you know this) that Jews were respected as a harmless curiosity (rather like the Indians), paleo-orthodox Catholics and (and depending upon which state we're talking about) Presbyterians in the 'out' box, and Muslims (and Buddhists and whatever) not in any box at all - theoretical abstractions of no practical consequence.
My response to both of my co-bloggers is regardless of HOW many non-Christians there were and WHAT the FFs "understood" about them, those "theoretical abstractions" were fundamental to their ideals and had serious practical consequences, not necessarily during the Founding era, but during the years to come. Those "theoretical abstractions" are THE PRIME reason why in 2009 the current President could accurately say America is not a Christian Nation.
Let's turn to the Founding record for evidence. First we have Jefferson's Virginia Statute on religious freedom. A relevant part reads:
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, and whose powers he feels most persuasive to righteousness, and is withdrawing from the ministry those temporal rewards, which proceeding from an approbation of their personal conduct, are an additional incitement to earnest and unremitting labors for the instruction of mankind; that our civil rights have no dependence on our religious opinions, more than our opinions in physics or geometry;
Jefferson in his autobiography also makes clear who exactly was covered under the act:
Where the preamble declares, that coercion is a departure from the plan of the holy author of our religion, an amendment was proposed, by inserting the word "Jesus Christ," so that it should read, "a departure from the plan of Jesus Christ, the holy author of our religion;" the insertion was rejected by a great majority, in proof that they meant to comprehend, within the mantle of its protection, the Jew and the Gentile, the Christian and Mahometan, the Hindoo, and Infidel of every denomination.
That is a classic sentiment of religious liberty and equality. You are free to practice your religion and you have equal rights as a citizen under the law regardless of your religion. If VA were a Christian State or the US a Christian Nation, only professing Christians would be recognized as full citizens. Now Jefferson and Madison believed in this "Virginia model" on Church & State which was at the "left" end of the spectrum. The "right" end of the spectrum was the Mass. model which permitted more integration of Church & State, indeed a mild religious establishment (which Mass. had until 1833). George Washington and John Adams are the most notable representatives of the Mass. model (yes, I know GW was from VA).
BUT, here's the kicker -- the Mass. model is still compatible with the notion of religious liberty and equality for all. And I would argue a natural rights republic, what America is, (as opposed to a "Christian state") demands any kind of state establishment be done while respecting religious liberty and equality rights of all, including non-Christians who are by nature equal citizens according to key American Founding thought. So how is this done? George Washington so explained, giving his reasons for why he, unlike Jefferson and Madison, didn't oppose government funding teachers of religion.
I must confess, that I am not amongst the number of those who are so much alarmed at the thoughts of making people pay towards the support of that which they profess, if of the denomination of Christians; or declare themselves Jews, Mahomitans or otherwise, and thereby obtain proper relief. As the matter now stands, I wish an assessment had never been agitated, and as it has gone so far, that the Bill could die an easy death; because I think it will be productive of more quiet to the State, than by enacting it into a Law; which, in my opinion, would be impolitic, admitting there is a decided majority for it, to the disquiet of a respectable minority.
Did you get that? As equal citizens with natural rights under the law if one merely declared himself a Jew or a Muslim he would be entitled to "relief" from having to support a religion in which he didn't believe. If VA were a "Christian" state, the response would be Jews and Muslims are lucky to worship freely here (arguably a "Christian State" wouldn't let them) and "Christianity" is entitled to your tax dollars regardless.