What is a state religion?

AnswerThere are few "one religion" states. Even Iran, which is predominantly Shi'ite Muslim, does have its Summi Muslims, Mandaeans, Zoroastrians, Bahai and some Christians. Perhaps the closest to a "one religion" state in modern times could be Saudi Arabia. Of course, during the Middle Ages, in most of Western Europe, the Roman Catholic faith was obligatory.