What is the story's main concept?

A Study in Scarlet is known for its very pointed and explicit attack on organized religion; the Latter Day Saints are the villains, and very pernicious ones at that. Many of the characteristics of the Mormons limned by Doyle are sensational and exaggerated, and there have been several criticisms from past and contemporary reviewers of the book. It is unclear whether or not Doyle admitted any fault for his work's prejudicial attitude towards the Mormons, but this has not stopped some school boards from objecting to its placement on reading lists.

The dangers of organized religion

Arthur Conan Doyle was raised in a Catholic family but eventually chose to leave the faith. Later in life he immersed himself in Spiritualism. He was never a fan of organized religion, which is quite conspicuous in this work. The Mormons are terrifying villains. Their leader Brigham Young is young and fiery, possessed of arrogance and imperiousness. The Danite Band, or the Avenging Angels, terrorize anyone who dissents from the creed. The faith is characterized by secretiveness, oppression, corruption, bribery, hypocrisy, and violence. Anyone who is perceived as a blasphemer faces death or mysterious disappearance. There are rumors of murdered immigrants and kidnapped women. Doyle is suggesting that organized religion squashes independence, autonomy, and freedom of thought. Those who were persecuted can easily turn to persecuting those who they believe are threatening them.