His presence at the Diet of Worms in 1521 brought him in contact with Martin Luther who turned him into a devout Lutheran. His succession in the elective kingdom of Denmark was not without trouble. The Danish cities were Lutheran but the Catholic nobility and farming communities preferred his half-brother, Hans. After the Diet at Viborg proclaimed Christian king, he had to wage what was known as the Count's War before being acknowledged. Supported by German mercenaries, the Catholic church was persecuted and Lutheran Protestantism was established as the State Religion. 
Emperor Charles V tried to interfere by encouraging his nieces, the daughters of the imprisoned King Christian II of Denmark, to contest the throne. All was resolved by the Treaty of Speier in 1544 when the Dutch were allowed entry into Danish and Norwegian harbours on payment of harbour dues. After all, Charles V was also Lord of The Netherlands. 

Another result of the Treaty was some relief of the harsh conditions of imprisonment of King Christian II who, in 1546, renounced his and his descendants' right to the throne. Christian II outlived Christian III by only twenty-four days, Christian III dying on 1 January 1559 at Coldingen and Christian II on 25 January 1559 in his prison at Kalundburg. 
 

Source: Leo van de Pas