The Greek Orthodox Church of Jerusalem (Arabic: كنيسة الروم الأرثوذكس في القدس Kanisatt Ar-rum al-Urtudoks fi al-Quds, literally Rûmi Orthodox Church of Jerusalem; Hebrew: הפטריארכיה היוונית-אורתודוקסית של ירושלים), officially called simply the Patriarchate of Jerusalem (Greek: Πατριαρχεῖον Ἱεροσολύμων, Patriarcheîon Hierosolýmōn), is an autocephalous church within the wider communion of the Eastern Orthodox Christianity. It is headed by the Greek Orthodox Patriarch of Jerusalem, the incumbent being Theophilos III since 2005. Christians believe that it was in Jerusalem that the Church was established on the day of Pentecost with the descent of the Holy Spirit on the disciples of Jesus Christ (Acts 2:1–41) and that the Gospel of Christ spread from Jerusalem. The church celebrates its liturgy in the Byzantine Rite, whose original language is Koine Greek, the original language of the New Testament, and follows its own calendar of feasts, preserving the Julian calendar (that is thirteen days behind the Western (Gregorian) calendar). It is also often called "Σιωνίτις Εκκλησία" (Greek: Sionitis Ecclesia, i.e. the "Church of Zion").
The number of Orthodox Christians in the Holy Land is estimated at about 500,000. A majority of Church members are Palestinians and Jordanians, and there are also many Russians, Romanians, and Georgians. The Church's hierarchy is dominated by Greek clergy, which in effect excludes the Arab majority from the Church's upper ranks. This has been a point of endless contention between Greeks in the patriarchate, who are backed in this regard by the Greek government, Israel and the Turkey-based Ecumenical Patriarch of Constantinople, and the native Palestinian clergy- some of whom seek to nationalize their Church's leadership. (see Arab Orthodox).
The headquarters of the Orthodox Church in Jerusalem is the Church of the Holy Sepulchre.
On the importance of Jerusalem, the Catholic Encyclopedia reads:
During the first Christian centuries the church at this place was the centre of Christianity in Jerusalem, "Holy and glorious Sion, mother of all churches." Certainly no spot in Christendom can be more venerable than the place of the Last Supper, which became the first Christian church.