In 1581 Ernest was elected the Bishop of Liège. When Archbishop Gebhard of Truchsess Waldburg converted to Protestantism in 1583, Ernest was elected Archbishop of Cologne on 22 May 1583. With Spanish and Bavarian troops he quickly drove Gebhard into Werl in the Cologne War ( or ""). In 1584 he was elected the Bishop of Münster, and also in that year the Papacy nuntiator was published. The Archbishopric of Cologne was important because it was one of the seven Imperial Electors of Holy Roman Emperor, and as three of them were already Protestant, if Cologne was ruled by a Protestant a Protestant Emperor could be elected.