Particularly, I'm interested in why the Founders chose to ban such tests federally, and not at the state level. One reason why the original Constitution didn't ban such tests at the state level is because, as with slavery, even if many or most of the Framers wanted to do this, the states probably wouldn't have ratified the Constitution if it meant abolishing their religious tests which most states at that time had.
Here is how Horwitz in today's post deals with what the Framers were concerned with in the context of state v. federal religious tests:
We might read that fact as having purely jurisdictional significance: the ratifiers were willing to see such a test at the federal level but not the state level. But I think it also adds to our understanding of the federal clause. It makes us understand just how revolutionary the federal clause was (a point several commenters have already recognized), and that may lead us to favor a somewhat narrow meaning for the clause. We might be more inclined to read the Clause as focusing more specifically and narrowly on the kinds of historical evils it was aimed against....
Earlier in the post Horowitz specified what that "historical evil" primarily was:
[T]he framers and ratifiers of the Religious Test Clause had a specific historical evil in mind when they crafted the Clause: that set of formally imposed oaths and very closely allied practices that were used to restrict office-holders in England to those willing to follow the doctrines of the established church. Second, we must consider the enforcement device by which these tests were imposed: the swearing of an oath, a device by which an individual risked his eternal soul or earthly honor should he swear falsely. The importance with which the members of the founding generation took oaths and the importance of their honor cannot be overstated.
While this much may be true, I think it misses a big reason why the key Founders may have wanted to abolish religious tests federally: Self-interest. They couldn't pass most of the religious tests that existed at the state level. Jefferson, Adams, and Franklin all disbelieved in the Trinity, Incarnation, Atonement, and Plenary Inspiration of Scripture. And Madison too was a secret closet theological unitarian. As I've written and published, I think Washington was as well, but given his reticence to speak on matters of his personal faith, the historical record is in dispute in this regard (though I think it points strongly towards his heterodoxy). Hamilton didn't convert to orthodox Christianity till the end of his life after he did his work "Founding" the nation. And James Wilson and G. Morris likewise were "theistic rationalists"/"unitarians" like Franklin, Adams, Jefferson, et al.
While it may have been "unthinkable" to electable elect a Catholic or a Jew to federal public office back then and whereas most of the above listed Founders had some kind of nominal affiliation with a Church which professed orthodoxy, Washington, Adams, Jefferson, and Madison all were elected President. The fact that these unitarian "infidels" didn't have to take an oath to Trinitarian Christianity or belief in the Divine Inspiration of the entirety of Scripture made their rise to office much easier. Perhaps they had that in mind when writing Article VI?
Franklin, though never elected President, was acting governor of PA and PA's Constitution, which he helped write had a religious test (I think not for the executive but for the representatives) which Franklin himself couldn't pass! Indeed, he wrote in a private letter (which I've blogged about here) explaining why he was against the religious test, in part because he couldn't pass it! (It required belief in the divine inspiration of the Bible and Franklin noted that parts of the Bible were impossible, in his opinion, to have been given by Divine Inspiration.) Intuitively, Franklin as a public official in PA, helped rid the offending clause.
If Franklin, Washington et al. were to construct a national religious test, it would be much simpler than what we see with most of them at the state level; it would require simple belief in a governing Providence who will ultimately reward good and punish evil; this is something all of them, at the very least, believed in. But they didn't even do that.
Finally, I think Horwitz should also consider what Gary North has written regarding oaths and covenanting. Most of the states, North argues, put forth religious tests in the context of making a covenant with the Trinitarian God of Christianity. The federal Constitution does not do this. And, if the leading Framers were not Trinitarians, they understandably would not want to covenant with the Trinitarian God of Christianity.
Horwitz seems more concerned with determining exactly what the clause forbids and dismisses such search for "expressive" meaning (in other words, regardless of what the clause exactly forbids, what does such a clause say about who we are/ought to be as a nation?). Lawrence Tribe, for instance, as quoted by Horwitz noted, the framers and ratifiers were moving to "prioritize the secular over the religious in the [federal] public realm." Kramnick and Moore, two Cornell scholars made a similar point in their book, The Godless Constitution. Gary North more or less agrees with Tribe and Kramnick and Moore that whatever the clause exactly forbids, it did make a meaningful, indeed revolutionary expressive statement about the federal government of the United States, who we are/ought to be as a "nation" and a "people." His point was that by purposefully not covenanting with the Trinitarian God of the Bible in the form of an oath to Him, and replacing such covenant with a "no religious test" clause, we are covenant breakers, and not a "Christian Nation." Even if courts in the 20th Century made technically "wrong" decisions secularizing the public square through the Establishment Clause, they did so, North argues, in large part because of the break with the orthodox Christian worldview the Founders set forth in Article VI's "no religious test" clause.
Such decisions, I suppose North would argue, were not within the "letter" but certainly were within the "spirit" of the Constitution.