Game concept · By God Alone
Christian
The game also links it as Christians, Christian's.
From the game's encyclopedia
An adherent of the Christian Religion.
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.
Christian is an entry of Crusader Kings III's in-game encyclopedia (By God Alone). The game explains it: "An adherent of the Christian Religion." The game also links it as Christians and Christian's. Patch 1.20.1.2, from game/common/game_concepts/pam_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
for a guide, post or blog
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.