During the 19th century, through a series of decrees, King Frederick William III united the Lutheran Church and the Reformed (Calvinist) Church in Prussia. The Prussian Union of churches (known under various names during its existence) was the church body which emerged through this union in 1817. It became the biggest independent religious organisation in the German Empire and later Weimar Germany, with about 18 million parishioners. The church underwent two schisms (one permanent since the 1830s, one temporary 1934–1948), due to changes in governments and their policies. After being the favoured state church of Prussia in the 19th century, it suffered interference and oppression at several times in the 20th century, including the persecution of many parishioners.
In the 1920s the Second Polish Republic and Lithuania, and in the 1950s to 1970s East Germany, the People's Republic of Poland, and the Soviet Union imposed permanent or temporary organisational divisions, eliminated entire congregations, and expropriated church property, transferring it either to secular uses or to different churches which were more favoured by the various governments. In the course of the Second World War the church underwent massive destructions of its structures by strategic bombing during World War II and by the end of the war many parishioners fled from the advancing Soviet forces. After the war complete ecclesiastical provinces vanished following the expulsion of most parishioners living east of the Oder-Neiße line.
The two post-war periods saw major reforms from within the church, strengthening the parishioners' democratic participation. In theology the church counted many renowned persons as its members – such as Friedrich Schleiermacher, Julius Wellhausen (temporarily), Adolf von Harnack, Karl Barth (temporarily), Dietrich Bonhoeffer, or Martin Niemöller (temporarily), to name only a few. In the early 1950s the church body was transformed into an umbrella, after its prior ecclesiastical provinces had assumed independence in the late 1940s. Following the decline of the number of parishioners due to the German demographic crisis and growing irreligionism the church body merged in the Union of Evangelical Churches in 2003. Many changes in the history of the church are reflected in several name changes.
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