The principle of "cuius regio, eius religio" says:

Cuius regio, eius religio 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. 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 Charles V agreed to accept this principle.