St Justin, in the chapter "The Characteristics of the Church" from his book The Orthodox Church and Ecumenism, writes:
Like the holy apostles, the holy fathers and teachers of the Church, with a godly wisdom and zeal like that of the cherubim and seraphim, confess the unity and uniqueness of the Orthodox Church. It is in the light of this that the fiery zeal of the Church Fathers in cases of separation and falling-away from the Church is to be understood, as is their strict treatment of heresies and schisms. The holy Ecumenical and Local Councils are, in this respect, of exceptional importance. According to their Christ-given spirit and assessment, the Church is not just a Church but the only Church. As the Lord Christ cannot have several bodies, there cannot, in Him, be several Churches. According to its theanthropic nature, the Church is one and one only, as the God-Man Christ is one and one only. Hence, the partition, the division, of the Church is ontologically and essentially impossible. The Church has never been divided, nor can it ever be, but fallings away from the Church have taken place and will again as the dry and barren branches fall away by themselves from the eternally-living theanthropic Vine, the Lord Christ (Jn 15:1-6). At various times, heretics and schismatics have separated and fallen away from the one and only indivisible Church of Christ, and have thus ceased to be members of the Church and parts of its theanthropic Body. The Gnostics first fell away, then the Arians, the Nestorians, the Monophysites and the Iconoclasts, the Roman Catholics, Protestants and Uniates, then, in their turn, the other adherents of the heretico-schismatic legion.