From the game's encyclopedia
The Christian Church is the ecclesiastical body that oversees and guides the followers of Jesus Christ, represented as a Situation that advances through a sequence of Chapters.
Its boundaries encompass every Archdiocese in the world; every Christian Ruler of County rank or higher with any part of their Realm inside an Archdiocese is a member, be they clergy or secular.
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.
The Christian Church is an entry of Crusader Kings III's in-game encyclopedia (By God Alone). The game explains it: "The Christian Church is the ecclesiastical body that oversees and guides the followers of Jesus Christ, represented as a Situation that advances through a sequence of Chapters." It covers The Christian Church Chapter, Chapter Type and Papal Bull. 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.