Apostolic succession is the tracing of a direct line of apostolic ordination, Orthodox doctrine, and full communion from the Apostles to the current episcopacy of the Orthodox Church. All three elements are constitutive of apostolic succession.
It is through apostolic succession that the Orthodox Christian Church is the spiritual successor to the original body of believers in Christ that was composed of the Apostles. This succession manifests itself through the unbroken succession of its bishops back to the apostles.
The unbrokenness of apostolic succession is significant because of Jesus Christ's promise that the "gates of hell" (Matthew 16:18) would not prevail against the Church, and his promise that he himself would be with the apostles to "the end of the age" (Matthew 28:20). According to this interpretation, a complete disruption or end of such apostolic succession would mean that these promises were not kept as would an apostolic succession which, while formally intact, completely abandoned the teachings of the Apostles and their immediate successors; as, for example, if all the bishops of the world agreed to abrogate the Nicene Creed or repudiate the Holy Scripture.
Orthodox teachings today are the same as that of the first apostles, though their mode of expression has adapted over the centuries to deal with heresies, changes in culture and so forth. This form of the doctrine was first formulated by St. Irenaeus of Lyons in the second century, in response to certain Gnostics. These Gnostics claimed that Christ or the Apostles passed on some teachings secretly, or that there were some secret apostles, and that they (the Gnostics) were passing on these otherwise secret teachings. Irenaeus responded that the identity of the original Apostles was well known, as was the main content of their teaching and the identity of the Apostles' successors. Therefore, anyone teaching something contrary to what was known to be apostolic teaching was not, in any sense, a successor to the Apostles or to Christ.
In addition to a line of historic transmission, Orthodox Christian churches additionally require that a hierarch maintain Orthodox doctrine as well as full communion with other Orthodox bishops. As such, the Orthodox do not recognize the existence of apostolic succession outside the Orthodox Church, precisely because the episcopacy is a ministry within the Church.