Up to the early 20th century, the Eastern Orthodox Church used the Julian Calendar universally, not accepting the calendric reforms of the Roman Pope Gregory XIII. 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 Russia in 1918 by a decree of the Council of People's Commissar's, 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 maintained the traditional Julian calendar Paschalion for calculating the date of Pascha.
(It must be noted that the Russian Orthodox Church and other Orthodox jurisdictions have not adopted the revised Gregorian calendar and still use the Julian. Thus, the majority of Orthodox Christians worldwide do use the Julian calendar for religious observation while using the modern calendar for daily life.)
The calendar change was not without controversy, and dissent arose from among both clergy and laity. In 1935, three Bishops from the Church of Greece returned their dioceses to the Julian calendar and consecrated four like-minded clergy to Episcopal dignity and then proclaimed that the 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.
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 factions. ROCOR recognized the other major faction in 1971. However, no official links exist between the Greek Old Calendarists and ROCOR at the present day.
In the present day, there are four major Old Calendarist divisions present in Greece, all of whom have parishes in many other countries.
Greek Old Calendarists prefer to adhere rigorously to traditional Greek Orthodox practices. While they are called (and might informally call themselves) "Old Calendarist", they 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". Many Old Calendarists maintain that they have "walled themselves off" from larger Orthodox jurisdictions to protect Orthodoxy from heretical innovations in practices and doctrine.