Fürstbistum Ermland
Old Prussians resisted the Teutonic Knights and received help from the Grand Duchy of Lithuania during the 13th century in their quest to free themselves of the military order. In 1525 Grand Master Albert of Brandenburg-Ansbach secularized the Order's Prussian territories into the Protestant Duchy of Prussia, a vassal of the crown of Poland. During the Reformation, Lutheranism spread throughout the territories, officially in the Duchy of Prussia and unofficially in the Polish province of Royal Prussia, while Catholicism survived in the Prince-Bishopric of Warmia, the territory of secular rule comprising a third of the then Diocese of Warmia. With Protestantism came the use of the vernacular in church services instead of Latin, so Albert had the Catechisms translated into Old Prussian.