From the game's encyclopedia
Doctrines could be considered religious laws that determine what is acceptable behavior within a Faith.
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.
Doctrines is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Doctrines could be considered religious laws that determine what is acceptable behavior within a Faith." It is part of Faith. It covers Gender Doctrines. The game also links it as Doctrine. 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.