Game concept
Fervor
From the game's encyclopedia
The Fervor of a Faith ranges from 0 to 100%. It measures the spiritual unity of the faith.
Fervor increases through aggressive expansion: County Conversion, winning Holy Wars, and resisting incoming holy wars.
Fervor decreases when the faith fragments: highly Divergent Rites sap fervor, and an unchecked heretical Rite depletes it further. Low Fervor makes it easier for rulers to create new Rites of their own.
Fervor also shapes how Rites spread within the faith: above 50%, the mainline Rite converts other Rites more easily; below 50%, non-mainline and heretical Rites gain the upper hand instead.
When Fervor drops to 40 or below, the faith becomes vulnerable to a Heresy outbreak.
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.
Fervor is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Fervor of a Faith ranges from 0 to 100%." It is part of Faith. 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.