Mormons not Christian? That's a fallacy of equivocation

The charge that Latter-day Saints aren't Christians often rests on fairly obvious fallacies of equivocation.

Such fallacies occur when an essential term is used twice (or more) in an argument, but in shifting senses.

One popular illustration of the fallacy was supplied by the late Irving Copi: "Criminal actions are illegal, and all murder trials are criminal actions; thus, all murder trials are illegal." Obviously, the term "criminal actions" is used with two different meanings, and the argument is bogus.

My favorite specimen, though, is this one, in which "all the world" is used equivocally:

I love you.

Therefore, I am a lover.

All the world loves a lover.

You are all the world to me.

Therefore, you love me.

How do those claiming that Mormons aren't Christians often commit a fallacy of equivocation? A common argument runs this way:

Mormons aren't Christians. Why? Because Mormons differ dramatically from the Christian mainstream, rejecting major doctrines (for example, the Nicene Trinity) that developed in the centuries after Christ.

Critics often accuse us of deceptively claiming to be traditional Christians, and puzzled outsiders sometimes ask why we claim to be Christians while rejecting certain doctrines and traditional creeds.

But we don't claim to be mainstream Christians, and these objections conflate or confuse "mainstream Christianity" or "traditional Christianity" or "historical Christian orthodoxy" with "Christianity" as a whole. They mistakenly assume that "Christianity" and "mainstream Christianity" are synonyms.

Obviously, the two are related. But they aren't the same — just as "box" and "cardboard box" aren't synonymous. (There are, after all, wooden, glass, metal, stone, plastic, and other kinds of boxes.) A cardboard box is a type of "box," but a person who doesn't want a cardboard box isn't necessarily rejecting boxes altogether. Likewise, a squirrel is a species within the larger class of mammals, and Catholicism and Methodism are species or types of Christianity. There are many types of mammal besides squirrels, many types of Christian beyond Catholics and Methodists.