Anyone who's read the very first Sherlock Holmes story, A Study in Scarlet, knows that the second half of the novel deals quite extensively with a sect that, at the time, was still somewhat in its infancy: The Church of Jesus Christ of Latter-day Saints, commonly known as the Mormons. A Study in Scarlet was published in 1887; the Book of Mormon was first published in 1830. So the founding of Mormonism was far closer in time to Doyle than Doyle is to modern readers.
If you have ever met any modern day Mormons (I've met quite a few in my life), you might find the sect as described in A Study in Scarlet vastly different from the people you've experienced. Portrayed as more or less fanatical minions of Brigham Young, the Mormons in the Holmes adventure are pretty grim figures. Hard to reconcile them with today's freshly scrubbed, shirt and tie wearing missionaries!
It's unclear to me how much research, if any, Doyle did into the sect he chose as his villains. According to Wikipedia, Doyle's own daughter said, "... father would be the first to admit that his first Sherlock Holmes novel was full of errors about the Mormons." (One obvious example is the reference to the Angel "Merona," a misspelling of the Angel Moroni from The Book of Mormon.) However, I don't know if he was that far off base at the time, considering how different Mormons of that era were from modern Mormons. I've done a pretty considerable amount of study of Mormon history, and it's pretty clear that Joseph Smith, Brigham Young, et al, were a great deal more fanatical than their modern descendants. Smith, after all, was killed by a lynch mob, after being imprisoned for his political activity. Brigham Young led a dedicated group of Saints out to the frontier country which later became Utah, a trip on which many died. And we've all heard about the polygamy that was common among Mormon leaders of the time. (To be fair, the modern Church of Jesus Christ of Latter-day Saints has declared that polygamy is completely incompatible with modern Church doctrine.) Doyle himself is reported to have said: "all I said about the Danite Band and the murders is historical so I cannot withdraw that, though it is likely that in a work of fiction it is stated more luridly than in a work of history." Debate still continues over A Study in Scarlet, and its depiction of Mormons.
The article linked below is an interesting piece on the controversy, written from an ex-Mormon's point of view:
Mormons believe A Study in Scarlet is inaccurate.
According to some reports Doyle later was repentant about his treatment of the sect in his novel. This article from The Salt Lake Tribune talks about Doyle's later dealings with the Mormons. Overall, I suppose the depiction of Latter-day Saints in A Study in Scarlet should be taken with a grain of salt. His treatment of the KKK in "The Five Orange Pips" still stands, though...
I'm a stay-at-home dad, and Director of Music Ministries at a United Methodist Church in Mt. Juliet, TN. And a longtime fan of Sherlock Holmes.