Throughout his life he remained one of Luther's most determined supporters; was with him at the Leipzig conference (1519), and the diet of Worms (1521); and was in the secret of his Wartburg seclusion.
Luther had meanwhile been concealed by his friends in the Wartburg, near Eisenach, where he busied himself with a new German translation of the New Testament, to be followed in a few years by the Old Testament.
He defeated Conrad near Frankfort in August 1246, but died in the following year at the Wartburg, when the male line of the family became extinct.
During Luther's stay in the Wartburg Jonas was one of the most active of the Wittenberg reformers.
He was fond of the society of men of letters, and Walther von der Vogelweide and other Minnesingers were welcomed to his castle of the Wartburg.