To expand upon the point made by
@Jürgen:
In 1527, Christian II made an attempt on conquering Norway with an army of German mercenaries: strong winds scattered much of his fleet, which meant that most of his heavy siege artillery was lost at sea. Consequently, he could not take the strong fortresses of the south, despite receiving the acclamation of most of the Norwegian nobility, which meant he had to negotiate with his uncle and usurpator, Frederick. Negotiations that led to his imprisonment in OTL. Let's say his fleet is a tad bit more lucky and he secures all of Norway and in the ensuing confrontation with Gustav Vasa and Frederick I manages to maintain that control, then his son John might wind up as king of Catholic Norway. However, this presupposes a lot of hand waving such as Christian’s own Lutheran tendencies and the successful defence against two much stronger and inherent neighbours.