Maybe it's the connection the Bible makes between fire and punishment. Maybe it's just that flickering, destructive flames work so well to signify a dangerous, unstable mind. Whatever the case, fire is often used as a motif in fiction for religious fanatics and extremists. It's pretty common to see such a character preaching in front of bonfires or braziers, or, in more extreme cases, burning sacrifices alive.
- In the 2010 Clash of the Titans, we see a rambling preacher burn his own hand in a brazier mid-sermon, showing both his total devotion to the gods and a totally unstable mind.
- The Night of the Hunter: Harry Powell gives his sermons in a small, dark church whose only source of light is a rather large brazier. Although a hypocrite and a thief, his fanatical faith does appear to be genuine.
- In The Hunchback of Notre Dame Frollo frequently references the fires of hell (including in his epic Villain Song, called "Hellfire") and attempts to burn Esmeralda at the stake, as well as burning down a lot of the city. He even meets his own death by fire, with a heaping side of irony, as he says "and he [God] will smite the wicked and plunge them into a fiery pit" before losing his footing on a gargoyle and falling into the "fiery pit" he's made of Paris.
- Anno Dracula: John Jago, an anti-vampire preacher, rants about fire and stakes (two of the only sure means of killing vampires).
- In the Discworld book Small Gods, a character from the fundamentalist nation of Omnia shows his God (who's not very attentive) a rather large furnace shaped like a bull, assuring said God that they use it for burning heretical material, not people. Turns out "heretical material" does indeed include people, Omnians just don't consider them thus.
- A subversion: Priests of the Red God in A Song of Ice and Fire. The best known one, Melissandre, plays it straight by being a total Knight Templar (but possibly a justified one) and has quite a penchant for burned offerings. However, other R'hllorists are just ordinary people in spite of the fire motif.
- Supernatural: Angels have a connection to fire: they burn out the eyes of anyone who sees their true form and their basic attack is to sear someone's insides with a touch. However, while they're all unquestionably trusting in God, it's not entirely fair to call them all fanatics.
- Warhammer 40,000: Sisters of Battle, Inquisitors, and other fanatical Imperial organizations have a posh for flamers and other fire-based weaponry.