From the game's encyclopedia
When the Fervor of a Faith falls critically low, a Heresy outbreak may erupt. Outbreaks come in three increasingly severe forms:
• An existing Rite becomes more Divergent and spreads further afield.
• A significantly Divergent new Rite appears and sweeps across the lands.
• For Christian Faiths, a historical heresy such as Bogomilism or Catharism appears.
A Faith stands at true risk of Heresy only once its Mainline Rite is in at least 100 Counties. Below that the two more severe forms cannot erupt at all, and even the mildest is far rarer.
Every outbreak ends in a unifying surge of Fervor.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Heresy is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "When the Fervor of a Faith falls critically low, a Heresy outbreak may erupt." It is part of Faith. The game also links it as Heresies and Heretical. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.