Orthodoxy's conviction that it is the true Church
of Christ on earth. It is actually because of its
possession of an uninterrupted historical and
theological continuity that it is able to make this
claim at all.
The Church, as we should expect of any historical phenomenon, has changed
and developed through the centuries. True enough. Still, the Church in its
essential identity - in its organic and spiritual continuity remains substantially
coextensive with the Church of the Apostles. It is, in effect, the living
continuation in time and space of the primitive Church in Jerusalem.

Orthodox Christianity begins with the first Pentecost in Jerusalem and the
outpouring of the Holy Spirit on Christ's small circle of disciples. It is then
that the Orthodox Church was born.

Equally crucial for the life of the Church was the formation of its
administrative structure. As a rule, the ministry of the Apostles was
itinerant, not stationary. After founding a community the Apostles would
depart for another mission, leaving behind others to administer the new
congregation and preside over the Eucharist and Baptism. In effect, a local
hierarchy developed whose functions were stationary, administrative, and
sacramental in contrast with the mobile authority of the Apostles. The
presiding officer of each community,  was the episcopos, or bishop, who was
assisted by priests and deacons.

By the early second century, this settled system with its threefold pattern of
bishop, priest, deacon was already in place in many areas. This structure
driven by the Holy Spirit, has helped the Orthodox Church expand.
The Origin of Orthodoxy