Reformation in Denmark–Norway and Holstein

During the Reformation, the territories ruled by the Danish-based House of Oldenburg converted from Catholicism to Lutheranism. After the break-up of the Kalmar Union in 1521/1523, these realms included the kingdoms of Denmark (with the former east Danish provinces in Skåneland) and Norway (with Iceland, Greenland and the Faroe Islands) and the Duchies of Schleswig (a Danish fief) and Holstein (a German fief), whereby Denmark also extended over today's Gotland (now part of Sweden) and Øsel in Estonia.

The Reformation reached Holstein and Denmark in the 1520s. Lutheran figures like Hans Tausen also known as the "Luther of Denmark" gained considerable support in the population and from King Christian II, and though next king, Frederick I, officially condemned the reformatory ideas, he tolerated their spread. His son Christian III officially introduced Lutheranism into his possessions in 1528, and on becoming king in 1536 after the Count's War, Lutheranism became official in all of Denmark–Norway. The Catholic bishops were removed and arrested, and the church was reorganized based on Lutheran church orders drawn up under the aegis of Luther's friend Johannes Bugenhagen in 1537 (Denmark–Norway) and 1542 (Holstein).

The Lutheran order established during the Protestant Reformation is the common root of the Church of Denmark, the Church of Norway, the Church of Iceland and the Church of the Faroe Islands. It also triggered Denmark's unsuccessful involvement in the Thirty Years' War under Christian IV, who led the defense of a Protestant coalition against the Catholic League's Counter-Reformation.