Overview
The Flame of Hatred is an unmarked secondary quest in The Witcher 3: Wild Hunt. It never appears in the quest log. It takes place on a street east of Hierarch Square in Novigrad.
The preacher
A preacher of the Eternal Fire is railing against mages, alchemists and above all witchers. When Geralt comes close, the sermon turns into personal insults. Geralt can ask how many lives the preacher has saved, threaten him into silence, or just walk away. Any choice ends the scene and the crowd disperses.
The consequence
If Geralt did anything but walk away, two guards stop him a few days later by a well in The Bits, between Hierarch Square and the Southern Gate. They accuse him of offending religious sentiment. Whether he hands over his swords or refuses, they bring up the preacher and attack. If you gave up your weapons, you will be fighting with fists and Signs, so remember to take your swords back from the bodies.
The XP from this quest is fixed and does not scale with level or difficulty.