The Constitution mentions religion in Article VI, section 3 ("no religious tests") and then again in the Establishment and Free Exercise Clauses. It's fairly uncontroversial among scholars that religious rights apply to more than just the "Christian" religion. Still, I have seen some originalists argue that the term "religion" meant Christian sects only. (Indeed, it's accepted "fact" among the Christian Nation crowd that "religion" which was protected by the Constitution only meant "Christianity".) Justice Rehnquist, although to my knowledge never held such rights should be restricted to Christians or Protestants, in his dissent in Wallace v. Jaffree seemed to imply that "religion," as originally understood, meant exactly this at least in the Establishment Clause (though like Scalia, I think he accepted that the Constitution's meaning can change over time if such meaning is consistent with the text of the Constitution and traditions of the nation; and indeed, extending "religious rights" beyond Christianity would qualify as such).
Rehnquist relied on Joseph Story's commentaries in that decision, and offered the following much cited quotation:
The real object of the [First] [A]mendment was not to countenance, much less to advance, Mahometanism, or Judaism, or infidelity, by prostrating Christianity, but to exclude all rivalry among Christian sects, and to prevent [p105] any national ecclesiastical establishment which should give to a hierarchy the exclusive patronage of the national government. It thus cut off the means of religious persecution (the vice and pest of former ages), and of the subversion of the rights of conscience in matters of religion, which had been trampled upon almost from the days of the Apostles to the present age. . . .
Now, this may well have been one of the "underlying purposes" of the First Amendment, but it would be a mistake to therefore conclude that the First Amendment, because one of its purposes was "to exclude all rivalry among Christian sects" protected "Christianity" only. For one, the text says nothing about "Christianity" but "religion," and the writings from the Founding era record make clear that everyone knew other "religions" existed besides Christianity.
Whether we call it "original intent" (at a general level) or "underlying principles" or "purposes" of the Constitution's text -- however useful these may be for explicating meaning or "filling in gaps" when needed -- such ought never be used to supersede the text of the Constitution. This is exactly what Randy Barnett warned against in this post where he wrote:
I do have one caution about Jack's [Balkin's] appeal to what he calls the "underlying principles" of the text. When the text is vague, appealing to the underlying principles to determine whether or not it covers a particular situation is appropriate and inevitable. But what is not kosher is to dive beneath the surface of the text to ferret out the "underlying principles" and then resurface somewhere else entirely. This is a standard technique by which the text itself can be replaced with the interpreter's version of the "underlying principles" that may even contradict the text itself.
What is appropriate is using the principles underlying the text to interpret its reach, when that reach is in doubt. In other words, when you dive beneath the surface to find the "underlying principles" you must reemerge in the text itself and apply the text (not the principle) to the facts of a particular case.
This post need not exhaust the public record to show that both the framers and the ratifiers of the Constitution, when they used the term "religion," knew that other religions besides the "Christian" one existed and were protected; trust me, they did. If they meant to give rights to the Christian religion only they could have worded the text that way, but they didn't. Though, I will show a little on the debates over Article VI's "no religious tests" clause. The ratifiers knew that by prohibiting religious tests for public office, non-Christians would be able to secure election and many were opposed to the Constitution for that very reason. But, since the Constitution ultimately was ratified, their view lost.
A North Carolina minister, for instance, in his state's ratification debate, noted that Article VI was "an invitation for Jews and pagans of every kind to come among us." At the Massachusetts convention, one speaker noted that unless the President was forced to take a religious oath, "a Turk, a Jew, a Roman Catholic, and what is worse than all, a Universalist, may be President of the United States."
Ultimately the original public meaning of the term "religion" in the Constitution demands that whatever "rights" or "prohibitions" attach to religion, attach to "religion in general" not the Christian sects only. Therefore, a view which I have seen put forth, similar to Rehnquist's dissent in Wallace v. Jaffree and Joseph Story's above quoted passage -- the Establishment Clause originally meant to prohibit a national Church plus discrimination or preference among the Christian sects, is untenable. If the EC demands any kind of sect neutrality, it must apply to all religions and therefore government could not promote Christianity in general while excluding "Mahometanism, or Judaism, or infidelity" from whatever program of aid is at issue. Perhaps government could promote "religion" over non-religion. But, if atheism or lack of religion qualifies as someone's "religion," (arguably it does), then government may not be able to do this either. Abolishing sect preference suggests a right to religious equality and religious equality may lead us down the road to government being forbidden from preferring one religion over another or preferring religion over non-religion (in other words, the post Everson line of cases).
Of course, if government were restricted from erecting a national establishment only it could promote Christianity over non-Christianity or one sect of Christianity over another (this is similar to what Philip Hamburger argues).
For a work which argues for a very strict reading of the Establishment Clause that avoids falling into the sect-preference pitfall and some of the problems with Philip Hamburger's "Separation of Church and State," (for instance that what Jefferson, Madison and the religious dissenters fought for in Virginia in the 1780s did not fall under the rubric of "separation of Church and State," an extremely contentious claim which Hamburger puts forth) see Vincent Phillip Munoz's "The Original Meaning of the Establishment Clause and the Impossiblity of its Incorporation" . University of Pennsylvania Journal of Constitutional Law, Vol. 8, 2006.
This passage on the meaning of "respecting an" impressed me:
The key to unlocking the meaning of the Establishment Clause lies in understanding the words "respecting an,"236 which were added by the joint committee. Then, as now, the present participle "respecting" means "with reference to, [or] with regard to."237 The added words reveal a precise intention -- to indicate that Congress lacked power with reference or regard to a religious establishment. By adopting "respecting an," the joint committee drafted a solution to the problem of how to craft language that would specify that Congress lacked power to legislate a national establishment or to interfere with existing state establishments (or lack thereof) without implicitly granting to Congress power to pass church-state legislation short of the stated prohibition.238 To restate the problem, if the committee drew a specific line that Congress could not pass (as proposed in the Senate), future congressional members might interpret their power to include everything short of that line. "Respecting an" offered a precise solution to this problem by indicating that Congress lacked power in the entire realm of religious establishments. Unlike the other First Amendment participles "prohibiting" and "abridging," which regulate but do not categorically deny Congress power, "respecting" indicates Congress's lack of jurisdictional authority over an entire subject matter. The Establishment Clause thus made clear that Congress lacked power to legislate a national establishment or to pass legislation directly regarding state establishments (or the lack thereof).239 Of course, Federalists in the First Congress, such as Roger Sherman, thought this was how the matter stood without an amendment.240 With the addition of "respecting an," Congress found language that did not affect the existing power of Congress (from the Federalists' viewpoint) yet would satisfy the fears aroused by Anti-Federalist criticisms that the Constitution threatened religious freedom. It was a remarkable feat of constitutional craftsmanship.
Note though, Congress wasn't just powerless over "religious establishments," but had no power over religion in general and arguably couldn't, as a general matter, promote religion for that reason. Munoz's paper avoids the pitfall of trying to read an empowerment over religion into the Constitution's text, which just isn't there.
However, if the Establishment Clause weren't incorporated, states could promote religion in general or a Christian or other sect in particular. For instance, Utah could promote Mormonism. Munoz's paper shows that while all of the Founders seemed agreed that Free Exercise for all religions must be guaranteed, they didn't likewise agree on Establishment policy. All three religion clauses -- the Establishment, Free Exercise and "No Religious Tests" -- were meant to secure the "unalienable rights of conscience." Jefferson and Madison endorsed the "Virginia" view or that strict separation of Church and State -- hence no government funds to promote religion -- was needed to secure those unalienable rights. Washington and Adams endorsed the "Massachusetts" view which thought a mild establishment didn't violate the rights of conscience. Hence even if the Free Exercise clause ought to be incorporated, because it is not clear that a mild establishment violates the unalienable rights of conscience and because of the precise way it is worded, the Establishment Clause does not neatly incorporate. The federal government, as originally conceived and persisting through the 14th Amendment, could make no establishment policy at either the federal or state level.
A couple of thoughts. First, even if it's true that certain types of establishments may not violate unalienable rights, certain types probably do. So even if states have leeway in their ability to establish and promote "religion," the rights of conscience will still demand *something* for those whose views aren't promoted by the state. This is what George Washington (and probably Adams and others who endorsed the "Massachusetts" view) believed. For instance, in commenting on Patrick Henry's bill to promote Christianity in general (which Madison remonstrated against and the contents of which were made illegal in Virginia according to Jefferson's Virginia Statute on Religious Freedom) Washington stated:
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.
In other words, he was against the bill not because he thought, in principle, it violated natural rights, but rather because it was "impolitic." But while he believed that state governments could, by right, support Christianity in general, because men of non-Christian religions have equal rights, they are, by right, entitled to "obtain proper relief."
And this, in turn is suggestive of a natural right to religious equality between not just the Christian sects, but for all religions. Whether when government "aids" various programs it must do so in a "religion blind" manner or could in fact "take cognizance" of religion -- promote a particular religious point of view, and grant equal accommodations, or "proper relief" (as Washington put it) to those whose views are not advanced -- is something I haven't entirely figured out. It's possible that, as Munoz and others argue, modern Establishment Clause jurisprudence greatly misreads the original record, though it's doubtful that the Court will unincorporate it anytime soon. Even if the Court did, I strongly believe, after Akhil Amar, the notion of religious equality or equal rights will vet much of what the Court currently has the incorporated Establishment Clause doing.
Finally, check out this enlightening debate featuring these very concepts with, among others, Michael Newdow and Phillip Munoz. On the matter of "under God" in the pledge, Newdow's position takes Madison's and Jefferson's "Virginia" position probably further than they would (but still to its logical conclusion) and argues religious equality/non-cognizance demands no "under God" in the pledge. After all, "under God" is not neutral or equal any more than saying "under no God" would be neutral or equal. Munoz, on the other hand, more in line with the Massachusetts view thinks it fine for government to promote theism over non-theism as long as atheists like Newdow could "obtain proper relief." In this case, it would be recognizing that atheists or other religious dissidents have an absolute right not to say "under God" in the pledge and suffer no penalty for it.