The current season of Game of Thrones saw the rise of a powerful sect in King's Landing, led by a down-to-Earth but ruthless leader bent on imposing religious law on Westeros's corrupt aristocracy.
In an interview with Entertainment Weekly, author G.R.R. Martin explains that, like so much else in the books and accompanying show, it's analogous to Europe on the cusp of the Renaissance.
“The Sparrows are my version of the medieval Catholic Church, with its own fantasy twist,” Martin told EW. “If you look at the history of the church in the Middle Ages, you had periods where you had very worldly and corrupt popes and bishops. People who were not spiritual, but were politicians. They were playing their own version of the game of thrones, and they were in bed with the kings and the lords. But you also had periods of religious revival or reform—the greatest of them being the Protestant Reformation, which led to the splitting of the church—where there were two or three rival popes each denouncing the other as legitimate. That’s what you’re seeing here in Westeros. The two previous High Septons we’ve seen, the first was very corrupt in his own way, and he was torn apart by the mob during the food riots [in season 2]. The one Tyrion appoints in his stead is less corrupt but is ineffectual and doesn’t make any waves. Cersei distrusts him because Tyrion appointed him. So now she has to deal with a militant and aggressive Protestant Reformation, if you will, that’s determined to resurrect a faith that was destroyed centuries ago by the Targaryens.”
The High Sparrow may be a hero, to some, but he'll never be the highest sparrow.
