thumb|Johannes Janssen.
Johannes Janssen (Xanten, 10 April 1829 – Frankfurt-am-Main, 24 December 1891) was a German Catholic priest and historian. He wrote an eight volume History of the German People, quoting many original sources.
Life
thumb|left|Johannes Janssen (1829-1891)
After graduating from the Rektoratsschule in Xanten (today's Stiftsgymnasium), he was educated at the universities of Münster, Leuven, Bonn and Berlin. It was while he was at the University of Louvain that he resolved to make the study of history his principal work. He became a teacher of history at the Gymnasium in Frankfurt-am-Main, a position he held up to the time of his death. Historian Ludwig von Pastor was one of his students.
Janssen had long been interested in entering the priesthood, but the delicate state of his health postponed that for some time. He was ordained priest at Limburg in March 1860. Not satisfied with attending to the ordinary duties of the classroom, Janssen devoted his spare time to historical research, the results of which were embodied in many learned volumes. In 1864-65 he spent several months in Rome, where he consulted the archives of the Vatican on matters relating to the Thirty Years' War. Janssen died in Frankfurt on Christmas Eve 1891. Wilhelm II, German Emperor sent a wreath for the coffin. In 1876-77 appeared a two volume biography of the writer and poet Count Friedrich Leopold zu Stolberg.
His great work is his Geschichte des deutschen Volkes seit dem Ausgang des Mittelalters (8 vols., Freiburg, 1878–1894), first suggested by Böhmer in 1853. He argues against Luther, Zwingli and the other Protestant reformers, and claimed that Protestantism was responsible for the general unrest in Germany during the 16th and 17th centuries. The author's conclusions led to some controversy, and Janssen wrote An meine Kritiker (Freiburg, 1882) and Ein zweites Wort an meine Kritiker (Freiburg, 1883) in reply to the Janssens Geschichte des deutschen Volkes (Munich, 1883) of Max Lenz, and other criticisms.
The Geschichte, which has passed through numerous editions, has been continued and improved by Ludwig Pastor,