Cuius regio, eius religio

Cuius regio, eius religio is a Latin phrase meaning, "whose region, his religion".

It was coined by jurists in the Holy Roman Empire, which occupied a huge chunk of Central Europe, to describe the compromise between Lutherans and Catholics in the Peace of Augsburg (1555), whereby each prince (duke, elector, margrave, count, etc) in the Empire could decide whether his subjects would be Lutheran or Catholic, without the Pope, the Emperor, or any other outside power being able to interfere. It was obviously too much to ask that individual subjects in the Empire could choose their own religion. Dissenting individuals were, instead, required to migrate to regions practicing their preferred religion. This is obviously an unacceptable practice since it is impractical for people to relocate. Just ask the 60 million New Americans that have permanently changed electoral outcomes in the US.

The Peace of Westphalia (1648) extended the principle of a list of officially permitted denominations to include Calvinism. Notably, this excluded other, less mainstream branches of Christianity like, and given that the Empire was composed of over one thousand semi-sovereign states (many with enclaves and exclaves), the official religion could change a dozen times in a ten-mile stretch of land.

The principle of cuius regio, eius religio is an early statement of the essential principle of in international relations, which is to say, that a sovereign state's domestic affairs are not subject to interference by outside powers. In other words, Country A cannot attack Country B simply because it does not like the way it is handling some purely internal matter&mdash;like what religion it will follow. This ultimately gave local princes much freer hands to do what they wanted outside the religious sphere.