Throughout Christian history, not only individual Christians but also entire Christian communities moved away from the unity with the Orthodox Church. Some of them have perished in course of history, while others have survived through the centuries. The most fundamental divisions of the first millennium, which have survived to this day, took place after the Third and Fourth Ecumenical Councils, when some Christian communities refused to accept their decisions. As a result, the Assyrian Church of the East and the Non-Chalcedonian Churches, including the Coptic, Armenian, Syrian Jacobite, Ethiopian and Malabar Churches, are separated even today. In the second millennium, the separation of the Roman Church was followed by internal divisions in Western Christianity, brought about by the Reformation, which resulted in the continual formation of different Christian denominations outside of communion with the Roman see. There were also breakaways from the unity with Local Orthodox Churches, including the Russian Church.
Delusions and heresies result from a person’s desire to assert himself and set himself apart. Every division or schism implies a certain measure of falling away from the plenitude of the Church. A division, even if it happens for non-doctrinal reasons, is a violation of Orthodox teaching on the nature of the Church and leads ultimately to distortions in the faith.
The Orthodox Church, through the mouths of the holy fathers, affirms that salvation can be attained only in the Church of Christ. At the same time however, communities which have fallen away from Orthodoxy have never been viewed as fully deprived of the grace of God. Any break from communion with the Church inevitably leads to an erosion of her grace-filled life, but not always to its complete loss in these separated communities. This is why the Orthodox Church does not receive those coming to her from non-orthodox communities only through the sacrament of baptism. In spite of the rupture of unity, there remains a certain incomplete fellowship which serves as the pledge of a return to unity in the Church, to catholic fullness and oneness.
The Orthodox Church cannot accept the assumption that despite the historical divisions, the fundamental and profound unity of Christians has not been broken and that the Church should be understood as coextensive with the entire “Christian world”, that Christian unity exists across denominational barriers and that the disunity of the churches belongs exclusively to the imperfect level of human relations. According to this conception, the Church remains one, but this oneness is not, as it were, sufficiently manifest in visible form. In this model of unity, the task of Christians is understood not as the restoration of a lost unity but as the manifestation of an existing unity. This model repeats the teaching on “the invisible Church” which appeared during the Reformation.
The so-called “branch theory”, which is connected with the conception referred to above and asserts the normal and even providential nature of Christianity existing in the form of particular “branches”, is also totally unacceptable.
Orthodoxy cannot accept that Christian divisions are caused by the inevitable imperfections of Christian history and that they exist only on the historical surface and can be healed or overcome by compromises between denominations.
The Orthodox Church cannot recognize “the equality of the denominations”. Those who have fallen away from the Church cannot re-unite with her in their present state. The existing dogmatic differences should be overcome, not simply bypassed, and this means that the way to unity lies through repentance, conversion and renewal. (Basic Principles of Attitude to the Non-Orthodox 1.13-15, 2.4-7) Source