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RGG1 was initially conceived in 1900 as a cross-disciplinary work which would incorporate the research of the new "history of religions school" and other liberal theological movements, as well as speak to the current situation of the church (hence the "gegenwart" (present) in its title. This "present" orientation was to be a corrective to the more academically oriented Realenzyclopaedie fuer protestantische Theologie und Kirche (3rd ed., 1896-1913), which formed the basis for the New Schaff-Herzog Encyclopedia of Religious Knowledge (1908-1912) [full-text online].
RGG2 (1927-1932) was a completely new work, covering a cross-section of German theological thought and reflecting the rise of neo-orthodoxy. Selected articles from it have been translated and published in Jaroslav Pelikan, Twentieth-Century Theology in the Making (3 vols., 1969-71).
RGG3 (1957-1962) had a more international character, with some contributors from throughout the world, but retained its fundamentally German Protestant orientation and its many articles of interest to German readers. The "present" focus continued (with articles on the ecumenical movement, the New Deal), and the Fraktur script of the first two editions was replaced by roman type.
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For a good review of the history of RGG in its various editions, see John Fitzgerald, et al., "Religion in Geschichte und Gegenwart: The Past and Present of a German Theological Dictionary," Religious Studies Review 27.4 (Oct 2001):319-329 [ DOI: 10.1111/j.1748-0922.2001.tb00374.x ]