Game concept
Religious Knowledge
The game also links it as Doctrine Knowledge, Knowledge.
From the game's encyclopedia
Characters do not know the full details of every religious practice.
Known tenets and doctrines can be used when creating a Divergent Rite, and may be adopted as Personal Tenets. Knowing all Tenets and Doctrines of a Rite makes conversion easier.
All Tenets and Doctrines within your Faith are known to you. New knowledge can be gained from relations like Spouses or Friends, Traveling, Activities, or the Study Rite Scheme.
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.
Religious Knowledge is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Characters do not know the full details of every religious practice." It is part of Rite. The game also links it as Doctrine Knowledge and Knowledge. Patch 1.20.1.2, from game/common/game_concepts/pam_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/pam_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.