get_random_faith_effect

get_random_faith_effect = {
	if = {
		limit = { $FORBIDROOTFAITH$ = yes }
		random_religion_global = {
			limit = { any_faith = { this != root.faith } }
			random_faith = {
				limit = { this != root.faith }
				save_scope_as = random_faith
			}
		}
	}
	else = {
		random_religion_global = {
			random_faith = { save_scope_as = random_faith }
		}
	}
}

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