Cuius regio, eius religio

"Cuius regio, eius religio" is a Latin phrase that means "whose land, his religion." It was a principle that emerged during the Reformation period in Europe, particularly in Germany. When the Protestant movement began challenging the Roman Catholic Church and their authority, various authorities and rulers were forced to decide whether to convert or preserve their religion.

Simply put, it means that whoever rules a specific region or area gets to decide what religion is practiced and followed there. For example, if a king is Catholic, everyone living in his kingdom must be Catholic too. Alternatively, if a ruler is a Protestant, then everyone in their kingdom must be a Protestant.

It was a way of promoting religious tolerance through the acceptance of the ruler's religion. However, this principle sometimes led to suppression of other religious beliefs, which caused conflict and persecution among different groups. Therefore, it can be seen as both a means of promoting religious freedom, as well as a tool of oppression or exclusion.