Hello Jukes,
Yes, I am afraid that Orthodoxy's claims extend beyond merely playing "an important role in Christianity", as does Rome's for that matter.
The Church is not simply an institution, although there is undeniably an institutional aspect to its being. She is a "mode of existence," a way of being. The mystery of the Church, even in its institutional dimension, is deeply bound to the being of man, to the being of the world to the very being of God. In virtue of this bond ecclesiology assumes a marked importance, not only for all aspects of theology, but also for the existential needs of man in every age.
In the first place, ecclesial being is bound to the very being of God. From the fact that a human being is a member of the Church, he becomes an "image of God," he exists as God Himself exists, he takes on God's "way of being." This way of being is not a moral attainment, something that man accomplishes. It is a way of relationship with the world, with other people and with God, an event of communion, and that is why it cannot be realized as the achievement of an individual, but only as an ecclesial fact.
However, for the Church to present this way of existence, she must herself be an image of the way in which God exists. Her entire structure, her ministries etc. must express this way of existence. And that means, above all else, that the Church must have a right faith, a correct vision with respect to the being of God. Orthodoxy concerning the being of god is not a luxury for the Church and for man: it is an existential necessity.
Because orthodoxy is a necessity for orthopraxy and for the very being of the Church, fundamental differences in doctrine must necessarily bring us to a choice. If Orthodoxy teaches the faith rightly, then we must believe that in the historical Orthodox Church there exists the full possibility of participating totally in the Church of God, and that in non-Orthodox Christian groups there are certain formal obstacles, varying in different groups, which, if accepted and followed by men, will prevent their perfect unity with God and will thus destroy the genuine unity of the Church.
At best, one can acknowledge these groups to be Christian in origin and heritage, but they cannot be understood to participate in the nature of the Church, seeing the contradictions that they hold regarding doctrine. This is not a judgment on the destiny of those souls residing in these organizations, but merely an observation regarding the institution they are involved in. God alone is in such a position to judge souls.
One could, I suppose provide scripture on the nature of the Church, but I believe both of us are familiar with the scripture I would present. It is agreed that the Church is one, and that the nature of this united Church involves a historical, institutional aspect to its existence. As I said, the reasoning is something that I believe we can both agree on, it is the facts of the case, whether or not this particular teaching or that specific doctrine is in fact erroneous.
This is not to say that the two theologies cannot learn something from each other. I would say the opposite actually, the two theologies, Eastern and Western, need to meet in depth, to recover the authentic patristic synthesis which will protect them from extremes which both are inclined toward. In the final analysis however, both of us agree that the Church is one and undivided and this is unavoidably an exclusionary position.
