I've toyed with the historical-theological notion that if one isn't an orthodox Trinitarian Christian, one isn't a "Christian." As a non-Christian, I am not personally wedded to it; I saw myself, rather, as giving due deference to the historical authorities -- be they Roman Catholic, Orthodox Christian, or Protestant -- in charge who defined Christianity according to its historical orthodoxy. I also found it a useful device when debating the "Christian Nation" issue because almost all of the proponents of the "Christian America" thesis define Christianity strictly according to orthodox doctrine. In short, the same folks who thunder "America was founded to be Christian Nation" are likely to turn around and assert "Mormonism isn't Christianity."
But I realize there is more than one way to define and understand "Christianity."
My co-blogger at American Creation Kristo Miettinen didn't like my assertion that "Christianity" could be defined synonymously with orthodox Trinitarian doctrine, at least from an historical perspective. He then challenged me to find respectable historians of Christianity who would, say, assert Arianism (or some kind of non-Trinitarian doctrine) was not "Christian." I think he understands there are plenty of theologians who will assert non-orthodoxy is "not Christianity," but rather than few respectable historians of religion will.
This is my first post attempting to name some names. Let me make some caveats. First, I'm not sure whether this will convince Mr. Miettinen for the sheer fact that there's going to be a semantical "out." I think almost everyone, whether they are historians, theologians or whatever, will concede things like Arianism, Socinianism, Mormonism, Jehovah's Witnessism, etc. to be "Christian heresies." In that case the term "Christian" is used as an adjective to modify "heresy," suggesting these heresies are part of the historical movement of "Christendom." I certainly don't disagree.
The second caveat I make is history doesn't occur in a vacuum. It's often used in political or cultural wars and this is CERTAINLY the case with the "Christian America" dispute. We literally are at the interdisciplinary crossroads of politics, history, and theology and you could throw in law and philosophy as well.
So when we see these historical scholars argue who and what is a Christian, you see them take cognizance of this political-historical-theological dispute and use lots of "if" qualifications. As in, "if we define Christianity this way, then...."
So here are some examples of notable historians who suggest, if it ain't orthodox, it ain't Christian. First is Paul F. Boller, PhD from Yale and Prof. Emeritus at Texas Southern University. His work has been published by among other places Oxford University Press. And he is considered the preeminent authority on George Washington's religion. Don't ask me why his book is out of print. All I know is that it is the most cited work by authoritative historians of GW. How he sums up Washington's creed in that book:
[I]f to believe in the divinity and resurrection of Christ and his atonement for the sins of man and to participate in the sacrament of the Lord's Supper are requisites for the Christian faith, then Washington, on the evidence which we have examined, can hardly be considered a Christian, except in the most nominal sense.
Boller then states "broadly speaking" Washington could be categorized as a "Deist" but notes GW nonetheless believed in an active Providential God. Boller also is open to categorizing GW as a liberal Protestant Christian of the unitarian bent or a nominal Christian.
Even Peter Lillback who holds Ph.D. from Westminster Theological Seminary in the history of theology, and wrote a book seeking to prove GW was a "Christian" not a "Deist," (wherein Boller was the main enemy) seemed to accept Boller's orthodox test for "Christianity." That is Lillback is not content to show GW was not a strict Deist (which I think Lillback clearly demonstrated), but was an "orthodox Trinitarian Christian."
Next, Gary Scott Smith, Chair and Professor of History at Grove City College, an evangelical school. He wrote a book on faith and the American Presidency published by Oxford University Press where he ENDORSED Dr. Gregg Frazer's categorization of "theistic rationalist" to describe the creed of America's key Founders. And again, perhaps Drs. Frazer and Smith find it impossible to separate their orthodox evangelical faith from their study of history and that shifts their perspective. Though, instead of trying to "claim" the FFs as "Christians," once they accept these FFs were not orthodox Trinitarians, they conclude the creed was not "Christianity" but some other theological system.
Again, I recognize this is a debatable contention; but OUP felt comfortable publishing this (what I consider an outstanding) book that made such an assertion. You can preview Dr. Smith's discussion of the theistic rationalism of the early Presidents in the link.
Next Dr. Peter Henriques, "Ph.D. in history from the University of Virginia in 1971, and he is professor of history, emeritus, from George Mason University." Dr. Henriques wrote a popular book published by UVA Press that also quotes Dr. Frazer and endorses the "theistic rationalist" categorization.
But Dr. Henriques doesn't say GW was NOT a Christian and even admits he could qualify as a "Christian" from a broad historical perspective. He cites Dr. Frazer's thesis AFTER noting that it's conservative evangelicals who most fervently try to claim GW as one of their own. And basically says, by your own standards GW wasn't a Christian, so perhaps you should look for a different term like "theistic rationalist." You can read his discussion of theistic rationalism in the google books preview here.
Also note Alan Wolfe's NYT review of Henriques' book:
Because today's religious right is determined to read the present back into the past, historians who write about faith and the founding find themselves on disputed ground. Nonetheless, both Henriques and Holmes are trustworthy guides. Henriques deals with Washington's life as a whole and spends only one chapter on religion. But he is fair-minded and thoughtful, and because he possesses no other agenda than a desire to uncover the real man, he is convincing when he concludes that "if one defines 'Christian' as the evangelicals do . . . George Washington cannot be properly referred to as a Christian."
Wolfe in that article also reviews David L. Holmes' book "The Faiths of the Founding Fathers," published by OUP. Holmes is the Mason Professor of Religious Studies at William & Mary. Note Holmes terms GW and the other key Founders as "Christian-Deists" or "unitarians" as opposed to the non-Christian Deism of Paine, Palmer, and Allen. However, he also writes in his book:
But if census takers trained in Christian theology had set up broad categories in 1790 labeled, "Atheism," "Deism and Unitarianism," "Orthodox Protestantism," "Orthodox Roman Catholicism," and "Other," and if they had interviewed Franklin, Washington, Adams, Jefferson, Madison, and Monroe, they would undoubtedly have placed every one of these six founding fathers in some way under the category of "Deism and Unitarianism." pp. 50-51.
Finally, here is Stephen Waldman on the Faith of the key Founding Fathers. Waldman is not a professional historian, but a journalist. However, historians Joseph J. Ellis, Walter Isaacson, and Mark Noll endorsed his book as did political figures William Bennett and George Stephanopoulos.
Here is how Waldman described the faith of the key FFs on a blog:
As for their religious beliefs, someone in the comment thread said I was being incoherent or contradictory by saying the Big Five (Franklin, Adams, Jefferson, Washington & Madison) were neither Deists nor orthodox Christians. Again, we’re viewing this through a somewhat warped lens. “Deist” and “Orthodox Christian” were not the only two spiritual choices. For one thing, each Founder was slightly different from each other, and changed throughout their lives. But if I had to pick a religion, I’d say they were sort of militant Unitarians. In other words, they had rejected or become uncomfortable with key parts of Christian doctrine and institutional behavior but they did believe in an active God, who intervened in their lives and the lives of the nation.
Note, he says something similar on p. 193 of his book. Though he clarifies with "it depends on how we define the term, but if we use the definition...offered by those who make this claim [that the FFs were Christians] -- conservative Christians -- then the Founders studied in this book were not Christians....If they must wear labels, the closest would be Unitarian."