Cuius regio, eius religio is a phrase in Latin that means, "Whose the region is, his religion." The principle was as old as state Christianity, established in Armenia and in the Roman Empire by Emperor Constantine.
In the Reformation, the old principle gained new life. It was the terminology used in the Peace of Augsburg (1555) (q.v.) to determine the religious makeup of Germany in a compromise between Lutheran and Catholic forces. The Peace offered imperial confirmation of the principle that had been promulgated in the Confession of Augsburg in 1530. The principle of the Augsburg Diet meant that the territorial princes and free cities gained the freedom to prescribe local worship, the right to introduce the Lutheran faith (the jus reformandi), and equal rights in the Holy Roman Empire with Catholic states. No agreement was reached on the question of whether Catholic bishops and abbots who became Lutheran should lose their offices and incomes, until this provision had been inserted by imperial decree.
However, the ideal of individual religious tolerance on a national level was not addressed: neither the Reformed nor Radical churches (Anabaptists and Calvinists being the prime examples) were protected under the peace. Many Protestant groups living under the rule of a Lutheran prince still found themselves in danger of the charge of heresy. Tolerance was not officially extended to Calvinists until the Treaty of Westphalia in 1648.
