What Is A Jack Mormon?
The OED defines "jack mormons" under variants of the noun "jack":
36. Prefixed to another noun denoting a person, a thing personified, a trade, or a quality, so as to form a quasi-proper name or nickname, often applied familiarly or contemptuously; as Jack Blunt (a blunt fellow), (the ‘Boots’ at an inn)[...] jack-gentleman, a man of low birth or manners making pretensions to be a gentleman, an insolent fellow, an upstart; so jack-gentlewoman, rare; Jack Mormon, a non-Mormon on friendly terms with Mormons; also, a nominal or backsliding Mormon;
And provides the following examples from the term's history:
*Jack Mormons, and sympathizers abroad may croak and groan over the poor Mormons.
1846Jack-mormon [see BIG-HEAD3b]. 1890Congress. Rec. 2 Apr. 2941/2 In our country we have a genus homo called ‘Jack-Mormon’,..a class of individuals who do not belong to the Mormon church,..yet who are ever found doing the bidding of Mormon priests. 1947Time 21 July 21/1 The number of backsliding ‘jack-Mormons’ is increasing.
Since the OED is primarily a British English dictionary, it doesn't include reference to the book that sets the word in its modern sense (the non-practicing Mormon) and disseminated it the broadest in American English, Edward Abbey's 1975 The Monkey Wrench Gang, and, more specifically, his description of Seldom Seen Smith at the opening of Chapter 3. Here's the first paragraph:
Born by chance into membership in the Church of Jesus Christ of Latter-Day Saints (Mormons), Smith was on lifetime sabbatical from his religion. He was a jack Mormon. A jack Mormon is to a decent Mormon what a jackrabbit is to a cottontail. His connections to the founding father of his church can be traced in the world's biggest genealogical library in Salt Lake City. Like some of his forebears, Smith practiced plural marriage. He had a wife in Cedar City, utah, a second in Bountiful, Utah, and a third in Green River, Utah--each an easy day's drive from the next. His legal name was Joseph Fielding Smith (after a nephew of the matyred founder), but his wives had given him the name Seldom seen, which carried.
I don't have much patience for jack Mormons who practice plural marriage these days, and I don't spend much time praying for "pre-cision type earthquakes," but that first sentences could go over on the right under my profile. It won't, of course--a bit long and so awkward to edit out the proper name.
To my knowledge, nobody today uses the term "jack Mormon" in the sense of the OED's primary definition; the "backsliding" sense has won out. What is interesting about the difference between the primary OED and the Abbey definitions is the sense one gets of the Church's former control in the Southwest. Anyone who lived in that area and traded with Mormons but wasn't a fully participating member could have been considered a "jack Mormon"--close enough to Mormon to be tarred with the same brush when the anti-pologamy campaign really heated up. Notice the date on the OED's Congressional Record quote? 1890, the year the Mormon-Congress stare-down resulted in a convenient revelation that plural marriage was no longer God's will on earth. By 1947, the date for the OED's citation documenting the shift in meaning, the church may have had overwhelming social and cultural power within Utah and surrounding areas, but the national stage saw the religion as a quaint frontier religion at best and a group of outback zanies at worst.
By Abbey's time, Utah governence was thoroughly secularized; deals between the Church and the state government were probably struck, but behind the scenes. The Church's bureaucracy had already gone corporate: it still owned a lot of land in Utah, but it had branched out its holdings into, among other industries, communications--and, strangely, Pepsico. It was possible to live in Nevada, Colorado, and even Utah and not to have to negotiate with the church's structure and power (its members and missionaries, perhaps less so). Mormons had already spread out across the country to live peaceful, middle-class lives in mixed communities--more virtuous and diligent lives, according to them, but your basic, non-frontier American lives, regardless.
These changes make the shift in definition of the term "jack mormon" possible. They also enable Abbey's romanticized frontiersman, Seldom Seen Smith. Smith is a fantasy-version of the early Mormon settlers in Utah, who damned the options society gave them at their time and struck out into the wilderness to live as they wanted. Smith's plural wives, which are rather disturbing to me on a literal level, could perhaps be read as signifiers of his adherence to cultural tradition rather than theological dogma and of his more general flouting of contemporary law. The book was, after all, written in 1975: Smith's polygamy is almost a quirky frontier spin on contemporary sexual practice. This blend of conservativism and self-determination---mixed with a heady degree of getting-away-with-shit---is what makes him a fantasy character. And that's before he starts to sabotage machinery and blow things up.
A jackrabbit to a cottontail, indeed!