The Reformation in the kingdom of Denmark–Norway was closely linked to the developments in the duchies of Schleswig and Holstein. The reason for this is not only to be found in the geographical position of the duchies between Germany and Denmark and their close political affiliation with Denmark in particular, but also in the fact that the evangelical movement made its earliest impact here.

In 1460 the nobility of the duchies had secured the constitutional position of Schleswig and Holstein as inseparable and indivisible. This decision was of considerable importance to the Danish king who, as duke of Schleswig and Holstein, was a vassal of the emperor, since Holstein constituted part of the Holy Roman Empire. However, even if the duchies remained a united administrative territory in a personal union with Denmark until 1848, dynastic interests meant that parts of the duchies continued to be ruled by younger members of the royal family. Thus, while Christian II was king of Denmark and Norway and duke of only parts of Schleswig and Holstein, his uncle resided in the castle of Gottorp and controlled substantial parts of the duchies. When, in 1523, the Danish lay and ecclesiastical nobility had forced Christian II into exile in the Netherlands, his uncle, the later Frederik I, was the obvious choice as king, strongly supported as he was by the aristocracy in Schleswig and Holstein.