The problematic “m” word in this week’s question is not “Mormonism” but “mainstream”: If you tell me where the mainstream is, I’ll tell you if the Mormons are in it.
There are surely more people who call themselves Christians in America than there are anything else, but the spectrum of their far more than 57 different varieties is so diverse, and there are now so many other religions in America, that it is hard to be certain precisely where the Mormons fit in. All that most Americans know about Mormons is that the men have lots and lots of wives, which isn’t true; but then, what most people know about most religions, including their own, isn’t true.
Most people have a mental map of the religious landscape of America just as screwy as the map that Saul Steinberg constructed his famous view of the world looking West from Manhattan, with New Jersey and points beyond it an increasingly distorted brown blur; on this map, the Mormons occupy roughly the space that Steinberg allotted to, say, Utah. Peripheral people such as Jews, not to mention American Muslims, probably have a more realistic map in their heads than “mainstream” Christians have, for they are at least well aware that, whoever it is that occupies the Archimedean point from which the Steinberg snap-shot is taken, it’s not them.
For there really is no mainstream; there are just larger or smaller by-waters. And among those by-waters the Mormons might be grouped with the religions whom most Americans generally define, theologically speaking, as “those-people-who-knock-on-your-door-and-give-you-pamphlets,” or locate somewhere between the horse-drawn Amish and the covens of New-Age Pagans. Indeed, most people probably regard Mormons as closer to the imaginary epicenter of the unofficial ecclesiastical map than some of the even more exotic religions like Zen Buddhism or Tantrism.
So, on the one hand, Americans think that Mormons are a lot weirder than they really are, but, on the other hand, Americans have become more accepting of religions that they regard as weird (there’s a word for this: pluralism). But, alas, the two options posed by the question are hardly mutually exclusive: if Mormonism has in fact come to be regarded as a part of the mythical mainstream, that might make people all the more suspicious of it.
The history of religions has demonstrated time and time again that dominant religions that are disdainfully tolerant of small religious movements as long as they remain small become vengeful, jealous gods when the movements become fruitful and multiply. It’s more peaceful back in the by-waters.