This article relies largely or entirely on a single source. (November 2021)
True Orthodox church, True Orthodox Christians, True Orthodoxy or Genuine Orthodoxy, often pejoratively "Zealotry", designates groups of traditionalist Eastern Orthodox churches which have severed communion since the 1920s 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.
The reformed church calendar was adopted by the mainstream Eastern Orthodox churches of Greece and Romania in 1924. At the moment of this adoption, True Orthodoxy began as Old Calendarism. True Orthodox were only laypeople and monks until 1935 when three bishops of the Church of Greece joined the movement in Greece; in 1955, one bishop of the Romanian Orthodox Church joined the movement in Romania. In the Soviet Union, the True Orthodox began in 1927-8 when some Eastern Orthodox Christians, among which some were "senior and respected bishops", severed communion with the Moscow Patriarchate.
The True Orthodox movement remained united in Romania. However, in Greece in 1937 the Greek Old Calendarists "divided"; the reason for their division is a disagreement on whether the sacraments performed by members of churches which have adopted the reformed calendar are valid or not.
In 1971, the Russian Orthodox Church Outside of Russia tried to unite the factions of Greek Old Calendarists, but failed. In 1999, the most important groups of Greek Old Calendarists were the Chrysostomites, the Matthewites, and the Cyprianites.
After the ROCOR opened its first parishes in 1990 in Russia, many Christians from the Catacomb Church joined them.
The True Orthodox churches are "fully [Eastern] Orthodox in dogma and ritual".
There is no single denomination nor organization called the "True Orthodox Church" nor is there official recognition among the "True Orthodox" as to who is properly included among them.
Denominations that are usually included in the True Orthodoxy are:
Many True Orthodox synods do not publish information concerning other synods, citing limited interest outside of their locality. Some are open to dialogue, whereas some hierarchies are more insular and prefer to keep to themselves. For example, the Russian Orthodox Autonomous Church (ROAC) through the late Metropolitan Valentine, stated informally that they no longer actively seek to join other True Orthodox churches, but would not refuse incoming dialogue.
In 1999, it was estimated that "[t]here are probably over one million Old Calendarists in Romania, somewhat fewer in Greece, and considerably fewer in Bulgaria, Cyprus, and the [Eastern Orthodox] diaspora."
Those who consider themselves a part of this movement are a minority of those who consider themselves to be Eastern Orthodox Christians.