The Reformation in Denmark–Norway and Holstein was the transition from Roman Catholicism to Lutheranism in the realms ruled by the Copenhagen-based House of Oldenburg in the first half of the sixteenth century. After the break-up of the Kalmar Union in 1521/1523, these realms included the kingdoms of Denmark and Norway and the Duchies of Schleswig (a Danish fief) and Holstein (a German fief), whereby Denmark extended over today's Denmark, Iceland, Greenland, Faroe islands, Skåneland and Gotland in Sweden, and Øsel (Saaremaa) in Estonia.