ep3_missionary_effect

ep3_missionary_effect = {
	# Same religion or unreformed or Hasan, convert to the preacher's faith
	if = {
		limit = {
			OR = {
				$LOCATION$.religion = $PREACHER$.religion
				$LOCATION$.faith = { has_doctrine_parameter = unreformed }
				character:41702 ?= $PREACHER$
			}
		}
		$LOCATION$ = {
			set_county_faith = $PREACHER$.faith
			set_county_rite = $PREACHER$.rite
		}
	}
	# Preacher is Gnostic, convert the locals to a regionally appropriate form of Gnosticism
	else_if = {
		limit = {
			$PREACHER$ = {
				OR = {
					rite = { rite_has_tenet = tenet_gnosticism }
					faith = { has_doctrine = special_doctrine_is_gnostic_faith }
				}
			}
			$LOCATION$.religion = {
				any_faith = {
					main_rite = { rite_has_tenet = tenet_gnosticism }
				}
			}
		}
		$LOCATION$ = {
			religion = {
				random_faith = {
					limit = {
						OR = {
							main_rite = { rite_has_tenet = tenet_gnosticism }
							has_doctrine = special_doctrine_is_gnostic_faith
						}
					}
					$LOCATION$ = { set_county_faith = prev }
				}
			}
		}
	}
	# Otherwise, reduce popular opinion
	else = {
		$LOCATION$ = {
			add_county_modifier = {
				modifier = ep3_religious_doubts
				years = 15
			}
		}
	}
}

As written in game/common/scripted_effects/07_dlc_ep3_scripted_effects.txt.