Fr. Hilarion was one of the foremost defenders of the dogma of the Unity of the Church against the heresy of ecumenism in its earlier forms. Thus in January, 1917, in the course of a debate with Mr. Robert Gardiner, secretary of the Joint Commission of the World Conference on Faith and Order, a forerunner of the World Council of Churches, he wrote:

“I could ask you this question: Do you and I belong to the one Church of Christ? In answering it you undoubtedly would mention the insignificance of our dogmatic differences and the virtually negligible difference in rites. For me, however, the answer is determined not by considerations of dogmatic disagreements but by the fact on hand: there is no ecclesiastical unity in grace between us…

“The principal truth of Christianity, its great mystery – the Incarnation of the Son of God – is acknowledged by all Christian creeds, yet this alone cannot fuse them into one Church. For, according to the Apostle James (2.19), the devils also believe; as attested by the Gospel, they confessed their faith like the Apostle Peter did (Matthew 16.16; 8.26; Mark 1.24; Luke 8.28). But do they belong to one Church of Christ? On the other hand, the Church community undoubtedly embraces people who do not know the dogmas of the Council of Chalcedon and who are unable to say much about their dogmatic convictions…

“If the question of the belonging or non-belonging to the Church be formulated in terms of theological dogma, it will be seen that it even cannot be resolved in a definite way. Just how far should conformity to the Church’s ideas go in dogmatic matters? Just in what is it necessary to agree and what kind of disagreement ensues following a separation from the Church? How are we to answer this question? And who has so much authority as to make the decision stand? Perhaps you will point to the faith in the incarnate Son of God as the chief characteristic of belonging to the Church. Yet the German Protestants are going to argue against the necessity of even this feature, since in their religion there are to be found even such ministers who openly deny the Divinity of the Saviour.

“Christ never wrote a course in dogmatic religion. Precise formulations of the principal dogmas of Christianity took place centuries after the earthly life of the Saviour. What, then, determined the belonging to the Church in those, the very first, times of the historical existence of Christianity? This is attested to in the book of the Acts of the Apostles: ‘Such as should be saved were added to the Church’ (2.45[-47 red.]; 6[5? red.].13-14). Membership of the Church is determined by the unity with the Church. It cannot be otherwise, if only because the Church is not a school of philosophy. She is a new mankind, a new grace-filled organism of love. She is the Body of Christ. Christ Himself compared the unity of His disciples with the organic unity of a tree and its branches. Two ‘bodies’ or two trees standing side by side cannot be organically related to each other. What the soul is to the body, the Holy Spirit is to the Church; the Church is not only one body but also One Spirit. The soul does not bring back to life a member which has been cut off, and likewise the vital sap of a tree does not flow into the detached branch. A separated member dies and rots away. A branch that has been cut off dries up. These similes must guide us in a discussion of the unity of the Church. If we apply these similes, these figures of a tree and a body, to the Church, any separation from the Church, any termination of the unity with the Church will turn out to be incompatible with membership of the Church. It is not the degree of the dogmatic dissent on the part of the separated member that is important; what is significant in the extreme is the fact of separation as such, the cessation itself of the unity with the Church. Be it a separation on the basis of but a rebellion against the Church, a disciplinary insubordination without any dogmatic difference in opinion, separation from the Church will for the one who has fallen away have every sad consequence.
“Not only heretics but schismatics, too, separate themselves from the Church. The essence of the separation remains the same.”

source: www.orthodox.net/russiannm/hilarion-archbishop-and-hieroconfessor-of-verey.html