"Religion," not "the Christian Religion," just as the text of the Consitution says.
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, and among all current members of the Supreme Court, that religious rights apply to "religion" which includes all religions. So it puzzles me why the Christian Nation crowd argues that the "religion" which the Constitution protects only meant "Christianity."
If the Framers meant to endow the Christian religion only they could have worded the text that way (which is, by the way, how many states at that time worded their constitutions) but they didn't. The debates over Article VI's "no religious tests" clause particularly show that the ratifers knew "religion" meant all religions. Indeed, many cautioned against ratification because they knew that by prohibiting religious tests for public office, non-Christians would be able to secure election. 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."
North Carolina delegate Mr. HENRY ABBOT stated:
The exclusion of religious tests is by many thought dangerous and impolitic. They suppose that if there be no religious test required pagans, deists, and Mahometans might obtain offices among us, and that the senators and representatives might all be pagans. Every person employed by the general and state governments is to take an oath to support the former. Some are desirous to know how and by whom they are to swear, since no religious tests are required --whether they are to swear by Jupiter, Juno, Minerva, Proserpine, or Pluto. We ought to be suspicious of our liberties. We have felt the effects of oppressive measures, and know the happy consequences of being jealous of our rights. I would be glad some gentleman would endeavor to obviate these objections in order to satisfy the religious part of the: society. Could I be convinced that the objections were well founded, I would then declare my opinion against the Constitution....
In New York, the New York Daily Advertiser featured an anti-constitutional article which attacked Art. VI. The author stated the no religious test clause would allow for the election of
"Ist. Quakers, who will make the blacks saucy, and at the same time deprive us of the means of defence--2dly. Mahometans, who ridicule the doctrine of the Trinity--3dly. Deists, abominable wretches--4thly. Negroes, the seed of Cain--Sthly. Beggars, who when set on horse back will ride to the devil--6thly. Jews etc. etc."
Finally, the author noted since the President headed the military "should he hereafter be a Jew, our dear posterity may be ordered to rebuild Jerusalem."
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." Cornell professors, Isaac Kramnick and R. Laurence Moore, in The Godless Constitution note:
Amos Singletary, another delegate to the Massachusetts ratification convention, was upset at the Constitution's not requiring men in power to be religious "and though he hoped to see Christians [in office], yet by the Constitution, a papist, or an infidel was as eligible as they." (p. 32)
From FEBRUARY 4, 1788, MASS. STATE RATIFYING CONVENTION:
Major LUSK...passed to the article dispensing with the qualification of a religious test, and concluded by saying, that he shuddered at the idea that Roman Catholics, Papists, and Pagans might be introduced into office, and the popery and the inquisition may be established in America.
That is just a taste. There is much more. See these pages put together by Jim Allison. The case is indisputable: The ratifers knew that the term "religion" unqualified by "Christian" meant all religions. Thus, whatever "rights" or "prohibitions" attach to religion, attach to "religion in general" not the Christian sects only.
Finally each of the key Founders -- Washington, Adams, Jefferson, Madison, and Franklin -- not only knew the term "religion" meant all religions, but believed all religions were equally protected under the unalienable rights of conscience.
First Jefferson, commenting on his Virginia Statute on Religious Freedom:
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.
Madison helped to pass such bill into law in Virginia. And here he confirms Jefferson's account:
In the course of the opposition to the bill in the House of Delegates, which was warm & strenuous from some of the minority, an experiment was made on the reverence entertained for the name & sactity of the Saviour, by proposing to insert the words "Jesus Christ" after the words "our lord" in the preamble, the object of which, would have been, to imply a restriction of the liberty defined in the Bill, to those professing his religion only. The amendment was discussed, and rejected by a vote of agst (See letter of J. M. to Mr Jefferson dated ) The opponents of the amendment having turned the feeling as well as judgment of the House agst it, by successfully contending that the better proof of reverence for that holy name wd be not to profane it by making it a topic of legisl. discussion, & particularly by making his religion the means of abridging the natural and equal rights of all men, in defiance of his own declaration that his Kingdom was not of this world [my emphasis].
Next Franklin, who involved himself in supporting and even building churches. Yet, this unitarian supported Trinitarian churches because he believed in "religion" in general, so much so that he didn't distinguish between his friend George Whitfield's orthodox Christianity and Islam.
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.
“It has pleased the Providence of the first Cause, the Universal Cause, that Abraham should give religion not only to Hebrews but to Christians and Mahomitans, the greatest part of the modern civilized world.”
– John Adams to M.M. Noah, July 31, 1818.
But Adams defines "religion" much broader than the Abrahamic faiths. He also finds it in Hinduism:
Where is to be found Theology more orthodox or Phylosophy more profound than in the Introduction to the Shast[r]a [a Hindu Treatise]? “God is one, creator of all, Universal Sphere, without beginning, without End. God Governs all the Creation by a General Providence, resulting from his eternal designs — Search not the Essence and the nature of the Eternal, who is one; Your research will be vain and presumptuous. It is enough that, day by day, and night by night, You adore his Power, his Wisdom and Goodness, in his Works.”
-- To Thomas Jefferson, December 25, 1813
And in pagan Greek worship:
θέμίς was the Goddess of honesty, Justice, Decency, and right; the Wife of Jove, another name for Juno. She presided over all oracles, deliberations and Counsells. She commanded all Mortals to pray to Jupiter, for all lawful Benefits and Blessings.
Now, is not this, (so far forth) the Essence of Christian devotion? Is not this Christian Piety? Is it not an Acknonowledgement [sic] of the existence of a Supream Being? of his universal Providence? of a righteous Administration of the Government of the Universe? And what can Jews, Christians, or Mahometans do more?
…Moses says, Genesis. I. 27. [”]God created man in his own image.” What then is the difference between Cleanthes and Moses? Are not the Being and Attributes of the Supream Being: The Resemblance, the Image the Shadow of God in the Intelligence, and the moral qualities of Man, and the Lawfulness and duty of Prayer, as clear[l]y asserted by Cleanthes as by Moses? And did not the Chaldeans, the Egyptians the Persians the Indians, the Chinese, believe all this, as well as the Jews and Greeks?…I believe Cleanthes to be as good a Christian as Priestley.
-- To Thomas Jefferson Oct. 4, 1813
For more on Adams see.
Finally Washington. Note, these key Founders differed on establishment policy. Though they believed all religions were equally protected under the unalienable rights of conscience, Jefferson and Madison believed strict separation was needed to secure such rights, while Washington and Adams thought a mild establishment didn't violate equal rights. So 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."