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Justus Jonas (5 June 1493 - 9 October 1555) was a German Protestant reformer.
He was born at Nordhausen in Thuringia. His real name was Jodokus (Jobst) Koch, which he changed according to the common custom of German scholars in the sixteenth century, when at the University of Erfurt. He entered that university in 1506, studied law and the humanities, and became Master of Arts in 1510. In 1511 he went to Wittenberg, where he took his bachelor's degree in law. He returned to Erfurt in 1514 or 1515, was ordained priest, and in 1518 was promoted doctor in both faculties and appointed to a well-endowed canonry in the Church of St Severus, to which a professorship of law was attached.
His great admiration for Erasmus first led him to Greek, Hebrew and Biblical studies, and his election in May 1519 as rector of the university was regarded as a triumph for the partisans of the New Learning. It was not, however, until after the Leipzig Disputation with Johann Eck that Martin Luther won his allegiance. He accompanied Luther to the Diet of Worms in 1521, and there was appointed professor of canon law at Wittenberg by Frederick III, Elector of Saxony.
During Luther's stay in the Wartburg, Jonas was one of the most active of the Wittenberg reformers. Giving himself up to preaching and polemics, he aided the Reformation by his gift as a translator, turning Luther's and Melanchthon's works into German or Latin as the case might be, thus becoming a sort of double of both. Jonas also assisted Luther with his translation of the Bible into German. He was busied in conferences (including a prominent role in the Reformation conferences at Marburg (1529) and Augsburg (1530) and visitations during the next twenty years, and in diplomatic work with the princes. In the auturm of 1531, Jonas published a German translation of the Apology of the Augsburg Confession and in 1541 he began a successful preaching crusade in Halle, becoming superintendent of its churches in 1542. In 1546 he was present at Luther's deathbed at Eisleben, and preached the funeral sermon; but in the same year was banished from the duchy by Maurice, Duke of Saxony.