Religion is pretty much there in Westeros. The Faith of the Seven does have a pope — it's the High Septon — but we're seeing in the course of the books what’s happened. The High Septons who were in when the story begins were kind of corrupt High Septons who liked to sit around their palaces, eat chocolates, and diddle little boys. But in the process of that, they have been replaced by the character they call the High Sparrow, who is a reforming zealot with a very different and more fundamentalist attitude toward some of these beliefs. Cersei is discovering, to her dismay, that he can’t be treated the same way as the previous corrupt ones. […] There’s a lot of material in The World of Ice and Fire about what happened to the Faith when the Targaryens took over, and the war between Aegon’s sons and the Faith Militant, which leads up to why things are the way they are in the present day.
- George R.R. Martin, 92NY Plus (2014)
The Faith of the Seven is having what you could only call a reformation movement and has become very corrupt by the time A Game of Thrones opens. The High Septon, who is the equivalent of the Pope, is a fat, sensual man, and when that High Septon is removed then it’s time to elect a new pope. The common people are called “sparrows.” One of them, who is handpicked by Cersei, becomes High Septon, and, of course, he gets called the “High Sparrow.” Cersei and the lords of King’s Landing find they have a person who wants to serve the poor, who’s cleaning out the corruption. In order to get what she wants, she allows him to revive the Faith Militant. Hundreds of years before the Targaryen arrival, the Faith had military arms with which they enforced their will — essentially these fighting monks. By allowing the Faith to take up arms again, you’re seeing the recreation of these fighting forces of young men pledged to the personal defense of the Faith and the High Septon. Suddenly, there’s a whole new player in the game.
- George R.R. Martin, Game Of Thrones Commentary (2014)
The Sparrows are my version of the medieval Catholic Church, with its own fantasy twist. 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.
- George R.R. Martin, Entertainment Weekly (2015)
