True Orthodox church

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True Orthodox church, True Orthodox Christians,^[1] True Orthodoxy or Genuine Orthodoxy, often pejoratively "Zealotry",^[2] are groups of traditionalist Eastern Orthodox churches which since the 1920s have severed communion with the mainstream Eastern Orthodox churches for various reasons, such as calendar reform, the involvement of mainstream Eastern Orthodox Churches in ecumenism, or the refusal to submit to the authority of mainstream Eastern Orthodox churches. The True Orthodox church in the Soviet Union was also called the Catacomb Church; the True Orthodox in Romania, Bulgaria, Greece and Cyprus are usually called Old Calendarists.^[1]