Old Calendarists (Greek: Παλαιοημερολογίτες, Paleoimerologites) also known as Genuine Orthodox Christians, are groups that separated from the Orthodox Church of Greece or from the Patriarchate of Constantinople, precipitated by disagreement over the retention of the Julian Calendar.
Up to the early 20th century, the Eastern Orthodox Church used the Julian Calendar universally, not accepting the calendar reforms of the Roman Catholic Council of Trent (1545-1563) and the then-current Roman Pope Gregory XIII. (By then, the Orthodox Church had been out of communion with the Roman Catholic Church for several centuries.) Traditionally Orthodox Christian countries, including Russia, Greece and Romania, did not use the Gregorian Calendar for civil and governmental affairs up through the first decade of the 20th century. The Gregorian calendar was imposed in Communist Russia in 1918 by a decree of the Council of People's Commissars, but only on civil affairs. Greece did not adopt a civil Gregorian calendar until 1923. In 1924, the Synod of the Orthodox Church of Greece voted to accept an altered form of the Gregorian calendar that both maintained the traditional Julian calendar Paschalion for calculating the date of Pascha and adopted a different method from the Gregorian of calculating leap years, the so-called Revised Julian calendar.
The Russian Orthodox Church, by far the largest Orthodox jurisdiction, and a number of other Orthodox jurisdictions did not adopt the Revised Julian calendar. These jurisdictions, and the majority of Orthodox Christians worldwide, still use the Julian calendar for religious observation, although most of the countries where Orthodox Christians live have adopted the Gregorian calendar for secular purposes.
The calendar change was not without controversy. Dissent arose from among both clergy and laity. In 1935, three Bishops from the Church of Greece returned their dioceses to the Julian calendar, consecrated four like-minded clergy to Episcopal dignity, created the church of the "Genuine Orthodox Christians" (Greek: Εκκλησία των Γνησίων Ορθοδόξων Χριστιανών - Γ.Ο.Χ.), and declared and that the official Orthodox Church of Greece had fallen into schism. By 1937, the number of Old Calendarist Bishops had been reduced to four, and the movement split within itself over the question of whether or not Orthodox jurisdictions that had adopted the modified Gregorian Calendar were still Orthodox.
After initial success in attracting followers the popularity of Old Calendarism waned in Greece, where the Church of Greece is the official state church. The Old Calendarists were relatively more successful in the United States, where religion is not regulated by the state. Until the early forties, old-calendarists comprised a majority of the Greeks in the U.S. As new immigrants poured in from Greece after WW II, however, their percentage dwindled to a small minority.
The Old Calendarists went their own way without further recognition from the broader Orthodox communion until 1960, when the Russian Orthodox Church Outside Russia (ROCOR) consecrated new Bishops for one of the two major Old Calendrist jurisdictions. ROCOR recognized the other major jurisdiction in 1971. However, no official links exist between the Greek Old Calendarists and ROCOR at the present day due to ROCOR and the Russian Orthodox Church (Moscow Patriarchate) having announced their intentions to reconcile.
In 1998, plagued by moral and financial scandals, a group of bishops that had broken off from the Church of the Genuine Orthodox Christians in the United States were re-baptized and re-ordained by the Ecumenical Patriarchate and put under the leadership of the Greek Orthodox Church in the U.S. (which uses the Gregorian Calendar). In exchange, their few priests that went with them were accepted as Orthodox priests, and their churches were allowed to maintain their use of the Julian calendar. Later these parishes were then forced to switch to the Gregorian Calendar and what remained of their flocks returned to the GOC.
In the present day, there are three major Old Calendarist divisions present in Greece, all of whom have parishes in many other countries. The first one is simply called the GOC, another the "Matthewites" as they derive from Archbishop Matthew and the third and smallest are called the Makarians (formerly called the Lamians) who broke off from the GOC when some of their bishops were accused of immorality and called to ecclesiological court. Relationships between the Matthewites and GOC are warming but the Makarians are not accepted by either of these Orthodox Churches.
Greek Old Calendarists prefer to adhere to traditional Greek Orthodox practices. While they are called (and might informally call themselves) "Old Calendarist", many maintain that they have not separated over a mere calendar. Instead, the calendar is a symptom of what has been called "the arch-heresy of ecumenism". On the other hand, some other Old Calendarists insist that the calendar is a matter of dogma since the Gregorian calendar had been anathematized by many pan-Orthodox councils in the past. Some Old Calendarists maintain that they have "walled themselves off" from larger Orthodox jurisdictions to protect Orthodoxy from heretical innovations in practices and doctrine.
Other than the calendar issue, Old Calendarists generally maintain the rites and beliefs of the Church of Greece, although there are other important differences on Baptism and Monophysites. Each church rejects the leaders of the other; some accept baptisms, weddings and funerals performed by the other church. Some Old Calendarists have gone so far as to reject even the baptisms of any and all Orthodox Christians in communion with the Church of Greece. This position is based on the belief that only the Church has Sacraments, there are no heresies within the Church, and therefore, the State Church is without a true Baptism.