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An Old Calendarist is any Eastern Orthodox Christian who uses the historic Julian calendar (called the "Old Style Calendar", "Church Calendar" or "Old Calendar"), proposed by the Roman statesman Julius Caesar, and whose church body is not in communion with the Eastern Orthodox churches that use the New Calendar.
The "Old Calendarists" are to be distinguished from Eastern Orthodox Christians or Eastern Orthodox Church bodies which are on the Old Calendar. The latter use the historic Julian calendar cited above, but are in communion with the Eastern Orthodox Churches that use the New Calendar (the Revised Julian calendar). Thus, to be "Old Calendarist" or "Old Calendar" is not the same thing as being "on the Old Calendar". The Russian Orthodox Church, for instance, is not Old Calendarist (or Old Calendar), but it is on the Old Calendar. There are a great many Eastern Orthodox Christians who are (or who belong to Churches that are) on the Old Calendar, but far fewer in number are the Eastern Orthodox Christians who are Old Calendar or Old Calendarist. It also should not be confused with the Oriental Orthodox churches, all of which are either on the Old Calendar or use their own calendar, but who are not in communion with either the Old Calendarists or mainstream Eastern Orthodoxy, although they are currently engaged in ecumenical dialogue with the latter. Nevertheless, inside the Armenian Apostolic Church, the Armenian Patriarchate of Jerusalem uses the Old Calendar in contrast to rest of the Armenian Church, which adopted the Gregorian Calendar while no member of the Armenian churches are using today the Armenian calendar. The Indian Orthodox Church uses the Gregorian calendar along with their autonomous Syriac Orthodox counterparts in India, the Malankara Jacobite Syriac Orthodox Church, in contrast to the rest of the Syriac Church which uses its own calendar. The Ancient Church of the East (Julian Calendar) emerged as the result of a schism when Patriarch Shimun XXI Eshai introduced the Gregorian Calendar into the Assyrian Church of the East.
The Julian calendar, proposed by the pagan Roman general Julius Caesar, is commonly opposed to the Gregorian calendar introduced to Christianity by Pope Gregory XIII during the 16th century. An improved form of the Gregorian calendar, originally developed in 1785 and modified in 1923 by the Serbian astronomer Milutin Milanković, was first introduced by Greece for civil purposes only and later adopted by some Orthodox churches. In its present form it is known as the Revised Julian calendar and it will not diverge from the Gregorian until AD 2800 (see Greek Old Calendarists#History). A minority of Eastern Orthodox Christians regarded this as a surrender of the Eastern Orthodox Church to the Pope and continued following the old calendar. Some of these also broke communion with those who had adopted the new calendar, thus creating their own church, or denomination, which means in Latin "to take a new name".
This schism is the beginning of the Old Calendar Churches which suspended full communion or concelebration with other Eastern Orthodox churches ("New Calendarists") over the adoption by the latter of the Revised Julian calendar (called "New Calendar," although some churches did not specify the details of which New calendar they were adopting). This is the most common use of the term.
Those Orthodox Churches which remain in full communion with the New Calendarists and yet continue to use the Julian calendar include the Eastern Orthodox Patriarchate of Jerusalem, the Macedonian Orthodox Church, the Russian Orthodox Church, the Serbian Orthodox Church, and the Georgian Orthodox Church. (The Julian calendar is also used by the Russian Orthodox Church Outside of Russia which has reunited with the Russian Orthodox Church.) Mount Athos, subordinate to the Patriarchate of Constantinople, also follows the Julian calendar.
In recent years both Old Calendar congregations and monasteries, as well New Calendar congregations and monasteries inside the Churches on the Old Calendar have been accepted into the official churches maintaining their own Calendar. Even some Russian Old Believers groups have been accepted into the official Russian Church while keeping their own traditions.
- OrthodoxWiki article about the Old Calendarists
- The Ecclesiological Position of the Old Calendar Orthodox Church of Bulgaria, by Bishop Photii of Triaditsa