From the game's encyclopedia
The Great Schism is the moment the unified Chalcedonianism Faith breaks apart along its Roman and Greek Rites, splitting into Catholicism and Orthodoxy.
It threatens when The Christian Church Situation enters certain Chapters, and rulers across Christendom must pick a side. Unless the divisions are mended — or forced apart — through Decisions, the split becomes permanent when the chapter ends.
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.
Schism is an entry of Crusader Kings III's in-game encyclopedia (By God Alone). The game explains it: "The Great Schism is the moment the unified Chalcedonianism Faith breaks apart along its Roman and Greek Rites, splitting into Catholicism and Orthodoxy." The game also links it as Schisms. 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.