From the game's encyclopedia
A Faith is a specific denomination of a larger Religion. All characters and Counties have a Faith just as they have a Culture, and follow one of its Rites.
A Faith is defined by its Mainline Rite, whose Tenets and Doctrines represent the beliefs of the Faith as a whole. Other Rites may champion different beliefs, and the further they drift from it the more Divergent they become.
Each Faith also has a Fervor value which controls how vulnerable it is to Heresy, as Divergent Rites break away into Faiths of their own.
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.
Faith is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Faith is a specific denomination of a larger Religion." It covers Religion, Head of Faith, Fervor, Doctrines, Clergy and 8 more. The game also links it as Faiths and Faith's. 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.