Cuius regio, eius religio

Cuius regio, eius religio (Ecclesiastical Latin: [ˈku.jus ˈre.d͡ʒi.o ˈe.jus reˈli.d͡ʒi.o]) is a Latin phrase which literally means "whose realm, his religion" – meaning that the religion of the ruler was to dictate the religion of those ruled. This legal principle marked a major development in the collective (if not individual) freedom of religion within Western civilization. Before tolerance of individual religious divergences became accepted, most statesmen and political theorists took it for granted that religious diversity was wrong and/or weakened a state – and particularly weakened ecclesiastically transmitted control and monitoring in a state. The principle of "cuius regio" was a compromise in the conflict between this paradigm of statecraft and the emerging trend toward religious pluralism (coexistence within a single territory) developing throughout the German-speaking lands of the Holy Roman Empire. It permitted assortative migration of adherents to two religious groups, Roman Catholic and Lutheran, eliding other confessions.

At the Peace of Augsburg of 1555, which ended a period of armed conflict between Roman Catholic and Protestant forces within the Holy Roman Empire, the rulers of the German-speaking states and the Holy Roman Emperor, Charles V, agreed to accept this principle.