The Greek Orthodox Church of Jerusalem (, Patriarcheîon Hierosolýmōn; Kanisatt Ar-rum al-Urtudoks fi al-Quds, literally "Church of the Rûm Orthodox in Jerusalem"), also known as the Orthodox Patriarchate of Jerusalem, is an autocephalous Orthodox Church within the wider communion of Orthodox Christianity. Headed by the Patriarch of Jerusalem, it is regarded by Orthodox Christians as the mother church of all of Christendom. 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 and that the Gospel of Christ spread from Jerusalem. The Church celebrates its liturgy in the Byzantine Rite, whose original language is Koine Greek, 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" (, i.e. the "Church of Zion").