Cuius regio, eius religio is a Latin phrase which literally means "whose realm, their 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 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 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 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.

Lutheranism is a major branch of Protestantism, identifying primarily with the theology of Martin Luther, the 16th-century German monk and reformer whose efforts to reform the theology and practices of the Catholic Church launched the Protestant Reformation.