Cuius regio, eius religio is a phrase in Latin that means, "Whose the region is, his religion." In other words, the religion of the king or other ruler would be the religion of the people. 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 was granted new life. It was the terminology used in the Peace of Augsburg embodied in the treaty signed in 1555 between the forces of the Holy Roman Emperor, Charles V, and the forces of the Schmalkaldic League, 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.

Long before the 16th century, disparities between an official public cult espoused by the ruler and the private cult of the majority have had effects on the course of history. In Visigothic Spain, the rulers and the bishops they appointed were Arians, while most of the population had converted to Catholic Christianity in the 4th century. When Moslem raiding parties came from North Africa in the late 7th century, the Visigothic kingdoms crumbled swiftly. The disaffection of the population was a factor.

Similarly in 7th century Syria, there was little loyalty to the emperor in Constantinople, partly because of recent controversies over the nature of Christ. Syrians were treated as heretics.

State sovereignty, considered absolute and unquestioned until after World War I, was undeniably eroded in the later 20th century.