By 1555 the Holy Roman Empire had spent a generation failing to suppress the Lutheran territories by law and by war, and the settlement reached at the Diet of Augsburg accepted the failure. Each imperial estate would determine whether its territory followed the old religion or the Augsburg Confession, and subjects who could not accept their ruler's choice were granted the right to emigrate with their property.
The tag by which the principle is known, cuius regio, eius religio, whose realm, his religion, was coined by a jurist some decades later and is not in the text, but it describes the arrangement accurately enough. Religion became an attribute of territory, decided by whoever held it.
Two devices were added to stop the settlement unravelling in one direction. The ecclesiastical reservation provided that a prince-bishop who converted must resign his see rather than take the territory with him, which preserved the Catholic bloc in the imperial constitution. A separate declaration protected some Lutheran subjects in ecclesiastical territories, and its status was disputed almost immediately.
The peace was a genuine achievement and it held, imperfectly, for over sixty years. It was also the first admission in imperial law that Latin Christendom would not be reunited, made by people who had grown up assuming that it must be.