game_rule_randomize_faith_effect

game_rule_randomize_faith_effect = {
	#Build faith list
	every_religion_global = {
		every_faith = {
			add_to_list = faiths_list
		}
	}

	#Save all rulers in tiered lists
	every_ruler = {
		if = {
			limit = { highest_held_title_tier = tier_barony }
			add_to_list = random_faith_barony_list
		}
		else_if = {
			limit = { highest_held_title_tier = tier_county }
			add_to_list = random_faith_county_list
		}
		else_if = {
			limit = { highest_held_title_tier = tier_duchy }
			add_to_list = random_faith_duchy_list
		}
		else_if = {
			limit = { highest_held_title_tier = tier_kingdom }
			add_to_list = random_faith_kingdom_list
		}
		else_if = {
			limit = { highest_held_title_tier = tier_empire }
			add_to_list = random_faith_empire_list
		}
		else_if = {
			limit = { highest_held_title_tier = tier_hegemony }
			add_to_list = random_faith_hegemony_list
		}
	}

	#Apply random faiths top-down (for conversion trickle down)
	every_in_list = {
		list = random_faith_hegemony_list
		game_rule_randomize_faith_randomization_effect = yes
	}
	every_in_list = {
		list = random_faith_empire_list
		game_rule_randomize_faith_randomization_effect = yes
	}
	every_in_list = {
		list = random_faith_kingdom_list
		game_rule_randomize_faith_randomization_effect = yes
	}
	every_in_list = {
		list = random_faith_duchy_list
		game_rule_randomize_faith_randomization_effect = yes
	}
	every_in_list = {
		list = random_faith_county_list
		game_rule_randomize_faith_randomization_effect = yes
	}
	every_in_list = {
		list = random_faith_barony_list
		set_character_rite = liege.rite
	}
}

As written in game/common/scripted_effects/00_game_rule_effects.txt.