Athens - -Orthodox church leaders from around the world met in Greece on Sunday for the first such gathering in a millennium, but the absence of several branches of the communion threatened to mar the event.
The clerics celebrated Orthodox Pentecost in the town of Heraklion on the island of Crete, where a "divine liturgy" was performed as part of the assembly that will last until June 27.
Istanbul-based Ecumenical Patriarch Bartholomew called the so-called Holy and Great Council on issues including wedlock, fasting and united representation in dioceses in countries such as the United States and Australia historic.
Greek President Prokopis Pavlopoulos took part in the mass.
But the absence of Russian Orthodox leader Kyrill, who represents some 130 million faithful - half the world's Orthodox population - threatened to undermine the image of unity that the gathered leaders had sought to project.
Aside from Russia, the Orthodox churches of Bulgaria and Georgia were also absent. Both are considered close to Moscow.
The Patriarchate of Antioch is also staying away because of a spat with Jerusalem over the appointment of clerics in Qatar.
The last such meeting was in 1054 when Christianity split between Catholicism and Orthodoxy, in the so-called "great schism" - and working out the details of the new council has taken over 50 years.
The Orthodox communion has about 250 million followers worldwide and consists of 14 autonomous churches.
Shaken by the upheaval in the former Soviet bloc and the Middle East it is frequently plagued by national and political strife.
Pope Francis, in his traditional Sunday prayer on Saint Peter's Square in Rome, called for reconciliation within the Orthodox Church.