E.D. Kain asks: are Mormons Christians? It's a question worth discussing, but it takes a long time, since it hinges on what it "really means" to be a Christian, and it's hard to come to any agreement on exactly where to draw that line. Some Catholics still believe that Protestant churches are full of idolators, and you can still find Protestants who believe the reverse. You can't determine whether Mormons are "really Christians" until you know what it means really to be a Christian. There's a lot of self-interest in that question, since many people want to draw the real-Christian line so that a particular someone falls inside of it.
What's easier to determine, as a matter of history and institutional continuity, is the degree to which someone is an orthodox Christian. As I've argued before, orthodoxy has to do with the history of the church you're a part of, not the objective, eternal correctness of your beliefs:
"To put it simply, your denomination's relationship to the Nicene Creed will tell you a great deal about the sense in which you can call yourself an orthodox Christian. If your denomination rejects it entirely, then you probably can't claim orthodoxy. This is not necessarily a matter of eternal salvation, just a matter of the sensible application of a label. Take Abraham Lincoln, for example. He attended church, but he never joined one as an adult. So I think it's safe to say that there's not much of a positive case for calling him an orthodox Christian. Am I 'casting him out'? Not at all. I really have no way of knowing whether or not Lincoln was 'actually a Christian,' and it's probably fruitless to try and pin it down.
"Unfortunately, and as usual, there is grey area. One can be a member of an orthodox denomination and yet be publically heterodox in one's beliefs, or one can adhere to a different theological tradition than that of one's church."
Does anyone object to the claim that we can look at church history and figure out some loose boundaries for orthodox Christianity, and that Mormonism falls outside those boundaries?
For a fuller discussion of Mormon theology and its differences with orthodox Christian theology, see the articles by Bruce Porter and Gerald McDermott in the October 2008 First Things.