This is an interesting Letter !
Dr. Demetrios Tselengidis,
Professor at Aristotle University of Thessaloniki
Firstly, it is necessary to clarify that as Orthodox Christians we believe, in agreement with the Symbol of Faith (the Creed) of Nicene-Constantinople (381 AD), “in One, Holy, Catholic, and Apostolic Church”. According to the unbroken dogmatic consciousness of the Orthodox Church throughout the ages, i.e., according to her self-consciousness, this One Church is the Orthodox Church.
The confession of the Symbol that the Church is “One” means that this is the basic attribute of her self-identity. In practical terms this means that the Church is not able to be divided – to be split apart – because this is the Mystical Body of Christ. Christ as the Head of the Body of the Church is neither able to have many bodies, nor to possess a divided one. In the Body of Christ even death itself is defeated. As such, whoever is placed within this Body also remains alive in it by the divinely-working mystery: the keeping with love of the commandments. They pass over from biological death to the eternal and everlasting life of the Triadic God. Just as the branches of the vine are not able to live and to bear fruit if they are cut-off from the vine, so also the believer – or even entire communities of believers, regardless of their numbers – who are cut-off from the Church, are not able to exist in Christ, nor to introduce another Church [into existence].
Rest of the letter here:http://www.impantokratoros.gr/members-church-heterodox.en.aspx