Mormons were definitely demonized at the time. A Study in Scarlet (1887), the first Holmes story, was first published in the Strand Magazine. It was one of dozens of novels in this time period that depicted Mormons as predatory villains trying to trap virtuous young women into polygamy. These "dime novels" were almost never particularly accurate, but they did reflect a real sense at the time that Mormons were degraded, lustful fiends.

Let me back up just a bit and define "Mormons." We're talking here about the Church of Jesus Christ of Latter-day Saints, based in Salt Lake City, Utah. There were other "Mormon" churches that traced their roots to Joseph Smith, including groups in Wisconsin, Pennsylvania, and the largest branch after the LDS Church, the Reorganized Church in Missouri. Smith founded the church in 1830, and after his assassination in 1844 in Illinois, several splits occurred over the next several years that led to multiple groups claiming to be the one true successor to Smith.

The Reorganized Church in particular went to great lengths to distance itself from the Utah church and denounce polygamy. They taught for many years (erroneously) that Joseph Smith never practiced polygamy and did not introduce it to the church, but that it was Smith's LDS successor Brigham Young who started it all. Smith did practice polygamy, but did so secretly, and so it wasn't until 1852 when the LDS Church announced to the world that it believed in polygamy.

Like many groups that have an unusual belief or practice that is maligned for the practice, the LDS Church dug in its heels on polygamy. Leaders began to preach that polygamy was not only an acceptable practice, but was superior to monogamy. They taught that in order to achieve full exaltation in the highest kingdom of heaven, one must be a polygamist. Utah was isolated in the American West—the Mormons settled it in 1847, but the transcontinental railroad wasn't completed until 1869. Rumors from federal appointees would make their way east into newspapers about how Brigham Young, who was territorial governor, was running a theocracy in Utah (he basically was).

In 1857, Mormons in southern Utah massacred an immigrant train of around 120 men, women, and children from Arkansas at a place that was called the Mountain Meadows. This Mountain Meadows Massacre was quickly reported in newspapers across the country. The act only further isolated the Mormons from the country and reinforced the view of them as a strange, bizarre people.

Cartoonists lampooned Brigham Young and the Mormons as oddities. Popular images showed Young in bed for dozens of women. They portrayed Mormons as menacing. After the Civil War, more attention was paid to the church and the federal government began to intervene. They passed anti-polygamy legislation, and they passed legislation restricting emigration of Mormon converts from Europe.

It's against all this backdrop that we get the appearance of these dime novels portraying Mormon men—especially Mormon missionaries—as predators. They often depict an innocent, naive young woman who falls under the Mormon man's spell. She is kidnapped or coerced into traveling to Utah. She must be rescued by a daring suitor or some other hero.

One of my favorites, for those of you familiar at all with Salt Lake City, has a climactic chase where the kidnapped woman climbs to the tallest spire of the Salt Lake Temple and leaps into the Great Salt Lake to escape her captors. The Great Salt Lake is at minimum 15 miles from the Salt Lake Temple. But none of that mattered to readers who had never been to Salt Lake and of course, given what they had heard, had no intention of going.

These dime novels were popular and most were of poor quality, but "A Study in Scarlet" stands out. Most people who dive into Sherlock Holmes and decide to read them in chronological order of publication are surprised to discover that the world's most famous detective, and one of the world's most famous fictional characters ever, got his start in an anti-Mormon dime novel.

The first half of the novel includes Holmes tracking down a killer, but the second half is dedicated to the Latter-day Saints in Utah (again, this surprises first-time readers), who come across a man and a girl in the desert and rescue them. The man and girl live in Utah, but when the girl comes of age, she is expected to become the plural wife of one of the sons of a prominent Mormon leader. They give her thirty days. But she has a suitor who intervenes to rescue her. I won't spoil the mystery or the story for those who would like to read it, but it's something else.

How accurate were these dime novels? They weren't. Apart from the many details they often invented or got wrong, they portrayed Mormons in a way that wasn't particularly fair to who they really were. Polygamy was not the utopia Mormons portrayed, of course. Men played favorites with their wives, and some women lived in different homes than their husbands, had little money, and lived in poverty. But on the flip side, women in Utah were granted the right to vote in 1870, long before nearly any other state in the Union. It was all, to say the least, complicated.

Dissenters and non-Mormons did live in Utah. They had plenty of complaints about LDS domination of politics and business, but by and large, they lived peacefully. In 1870, the "Salt Lake Tribune" was founded, which stood in stark opposition to Young and LDS publications. It provided an outlet and news for non-Mormons.

As the federal government continued to crack down on polygamy, things became intolerable for the church. The Edmunds-Tucker Act (1887) was the last straw. The government threatened to seize church property, including temples. It had already disenfranchised polygamists and states like Idaho had implemented test oaths for anyone wanting to vote or run for public office. In 1890, the Woodruff Manifesto announced that Mormons would no longer enter into new plural marriages.

But the church continued to approve some plural marriages in secret. Rumors continued about these marriages and Mormons continued to be lampooned in the media. The advent of motion pictures allowed for a new kind of dime novel—the anti-Mormon movie. Films like "A Mormon Maid" (1917) and "A Victim of the Mormons" (1911) followed the basic plotline of the earlier dime novels. They portrayed Mormons wearing strange robes and clothing and trapping women into unwanted polygamous marriages.

But by then the church had truly abandoned polygamy with the second manifesto in 1904. Apostle Reed Smoot had been elected to the US Senate, and lengthy hearings were held in Washington, DC, to see if he should be granted his seat. Mormon church president Joseph F. Smith testified before Congress, albeit at times not very truthfully, about the continued practice of polygamy. Because of the attention paid to the church, Smith issued the manifesto and banned all future plural marriages.

I've covered a lot of ground, but for some good overviews of this, I recommend Sarah Barringer Gordon, _The Mormon Question: Polygamy and Constitutional Conflict in Nineteenth-Century America_ (Chapel Hill: University of North Carolina Press, 2002); Kathleen Flake, _The Politics of American Religious Identity: The Seating of Senator Reed Smoot, Mormon Apostle_ (Chapel Hill: University of North Carolina Press, 2004); Michael Austin and Ardis Parshall, _Dime Novel Mormons_ (Salt Lake City: Greg Kofford Books, 2017); and for portrayals of Mormons as savages and "other," see W. Paul Reeve, _Religion of a Different Color: Race and the Mormon Struggle for Whiteness_ (New York: Oxford University Press, 2017).

Edit: A few commenters have called out my language in describing the Mountain Meadows Massacre as something that made Americans see Mormons as odd and "strange." It's a fair criticism and I wasn't trying to minimize the horrific nature of the massacre; rather, I was trying to explain that Mormons were already viewed as a bizarre religious cult. There were rumors around "Danites"—Mormon assassins who would execute dissidents or those who tried to leave the faith.

The massacre itself was, if possible, worse than it sounds. After a siege, Mormons separated the men from the women and children, had the women and children march a ways down a trail, and then they killed both groups. Later, Mormons tried to blame the massacre on Paiutes. While there were some Paiutes involved, the massacre was wholly the result of southern Utah Mormons surrounding the wagon train and then deciding to exterminate everyone except a handful of extremely young children—infants and toddlers, essentially. I'd recommend reading Juanita Brooks or Will Bagley on the massacre. Though I'm not persuaded by Bagley's assertions on Brigham Young, he is a brilliant researcher and writer, and a dear friend.

This thread shows how contested Mormon history is. I've already been asked if I'm LDS because my answer is perceived as too biased in favor of the Mormons. I'm not LDS, but I'm very used to the skepticism. Although Mormon studies is becoming more and more professionalized as an academic practice, it frequently inspires a very binary response. Latter-day Saints want to know, "Is this person for us or against us?" Conversely, those who have left the faith are, if possible, even more intense and skeptical—at least in my experience.

My use of terms like "complicated" and "messy" isn't an effort to whitewash or somehow defend the LDS Church, but it's to acknowledge that these things *are* messy and complicated. Even here on Reddit, which allows for much broader discussion than say, Facebook or Twitter, there are still limitations. I'd encourage anyone interested in a particular topic to PM and I'm happy to offer additional reading.

The early Mormon Church invested heavily in its proselytizing efforts and found its greatest success with missions to the United Kingdom. New converts were encouraged to take the arduous trip to America and across the plains to Utah as soon as they were able. Even now, descendants of immigrants from the British Isles form the largest source of the populations in Utah and Idaho .

As a consequence of these efforts, the British experience of Mormonism, which Conan Doyle might, in part, have been reacting to, was of young missionaries preaching to people who would subsequently disappear (to the U.S.) forever. Mormonism was commonly considered a cult and, of course, their practice of polygamy was well-known and widely denounced in Victorian England. Mormon missionaries were sometimes portrayed in the press and early films as Svengalis, who would mesmerize and brainwash their targets (typically young women).

Conan Doyle was known to be an avid reader and historians have suggested he based his opinions of Mormonism on several books published in England, including two by defectors from, and critics of the practice of polygamy: Fannie Stenhouse and Anna Eliza Young (one of the 55 wives of Brigham Young). These books describe a regime that was, in fact, seriously oppressive toward women. Other books commonly cited among his influences were authored by William Hickman, William Jarman and John Hyde.

William "Wild Bill" Hickman was a member of the Church, who claimed to have murdered several people at Brigham Young's direction. These included an extermination order against the Timpanogos tribe of Utah. He was later excommunicated when, according to his account, he refused to assassinate someone else at Young's order. Hickman then wrote a book admitting numerous murders, which was published under the title, Brigham's Destroying Angel: being the life, confession, and startling disclosures of the notorious Bill Hickman, the Danite chief of Utah.

Conan Doyle's first Holmes novel was clearly inspired, in part, by the Danites, a mysterious fraternal organization of Church members. The Danites were organized as a vigilante group to fight in the 1838 Mormon War, and were sometimes referred to as Destroying Angels. There is little evidence of the group's continued existence after 1838, but they became the subject of myth both in and outside the Church. Named for one of the twelve tribes of Israel, the Danites were supposed to a cabal of Mormon leaders, who acted as secret enforcers for the Church. Brigham Young repeatedly denied their existence, as he did in June 1857, when he said publicly: "[people claim that the Danites] are in every town and city throughout the whole of the United States, and that their object is not known by the people. That they are all over the world; that there are thousands of them, and that the life of every officer that comes here is in the hands of the Danites. That even the President of the United States is not safe, for at one wink from Brigham the Danites will be upon him and kill him...It is all a pack of nonsense, the whole of it." Perhaps not surprisingly, such denials were not universally successful in suppressing the rumors.

Conan Doyle was swept up in and convinced by some strange, fad beliefs, such as spiritualism and the existence of fairies. In any case, he wrote A Study in Scarlet in three weeks, while he was also a practicing physician, suggesting it was not a heavily researched work, but sprang from his impressions of the Church formed from rumor and whatever prior reading he had done.

Conan Doyle defended his work against a backlash from Mormons by saying the kind of events he described (murder, kidnapping, secret surveillance...) were, "a matter of historical record," though he admitted the descriptions were, "lurid." His daughter would later say, "You know, father would be the first to admit that his first Sherlock Holmes novel was full of errors about the Mormons."

To conclude, and to answer your questions, I wouldn't say Conan Doyle had weird prejudices, but he was reflecting the unsorted mix of prejudice, myth and fact available to him at the time. Again, though clearly highly intelligent, he could be overly credulous at times. He was also a fiction writer, more concerned with delivering a sensational story than carefully recounting facts.

While there is always more to say, u/skedaddle u/YeeChangLee and a few others wrote answers a long while ago check it out

Further links: answer by u/manpace can be found here

More answers, (including a repeat by u/YeeChangLee) by u/Mr263414 and u/yodatsracist can be found here