The town was an important breeding ground for the reformation in Denmark, and as early as 1526 Christian introduced, as the duke of Schleswig-Holstein, the reformation in Haderslev, just eight years before he became King of Denmark.
Byen var et af arnestederne for reformationen i Danmark, for allerede i 1526 indførte Christian som hertug af Slesvig-Holsten reformationen i Haderslev, otte år før han blev konge af Danmark.