For more on this topic see "Know the Faith: A Handbook for Orthodox Christians and Inquirers" - https://store.ancientfaith.com/know-the-faith/
Introductory Note:
In our age, more than in any other, the crucial question for Christians is: “What and where is the Church?” This is the question of “ecclesiology” (that which pertains to the Church). Even after the Protestant Reformation many basic assumptions about what church is and how one is made a member of the church were left intact. One of these assumptions is that the Church is a tangible and visible body to which one must be joined in order to be considered a Christian. Another general assumption held by all was that one must hold to the whole body of doctrine and faith of a church in order to receive Communion. It was also held that the reception of Communion in a church makes one a member of that church and that there were required prerequisites for being allowed to Communion. Finally, the common belief among most Christians and Christian churches was that there can only be one Church from which a person is either joined to or separated from.
Yet today – as a justification for the existence of thousands of denominations – these ecclesiological assumptions have been abandoned and are no longer even part of the modern Christian worldview. In 1938 the Anglican, William Palmer, formulated the “Branch Theory” which understands the Orthodox, Catholic, and Anglican churches as three legitimate co-existing branches of apostolic Christianity. The theory breaks with the ancient ecclesiology since it suggests that these three branches make up the one body of Christ. As a way of justifying the separation of Rome from Orthodoxy the theory of the “two lungs” also appeared, which understands the Catholic and Orthodox churches as two lungs in the same (one) body. These theories are precursors to the modern idea that the Church can only be whole again if and when all Christian bodies are reunited.
Although not the original intention, these ideas have been promoted by the “ecumenical movement” which began after WWI.[1] Such theories are unacceptable for the Orthodox who follow the Saint’s teaching that the Church cannot be divided. Schisms from the Church do not deplete the fullness of the Church. Our confession of faith in “one, holy, catholic, apostolic church” means that the fullness of the Church is dependent upon God not man. The Church is “one” because God is one. She is holy because God is holy. She is complete (catholic) because He is catholic. Men can only be brought into communion with the one, holy, catholic, apostolic Church or separated from Her. This is the Faith of the Church.
What Makes a “Church”?
Roman Catholicism:
The Church is dependent upon the Pope, who is understood to be the successor of St. Peter:
“After his resurrection our Savior handed her [the church] over to Peter to be shepherded (John 21:27), commissioning him and the other apostles to propagate and govern her. . . . This church, constituted and organized in the world as a society, subsists in the Catholic Church, which is governed by the successor of Peter and by the Bishops in communion with him.”[2]
Classical Protestantism:
The Church is founded on a perceived “purity” of preaching and administration of sacraments.
John Calvin: “Wherever we see the Word of God purely preached and heard, and the sacraments administered according to Christ’s institution, there, it is not to be doubted, a church of God exists”
Evangelicalism:
The Church is founded upon the Bible and the individual believer’s faith. One Evangelical has defined it in the following way: An “Evangelical church is a Protestant Christian group that believes in four principles: being “born again” or conversionism, activism (sharing of the gospel), crucicentrism (Jesus death) and Biblicism (teaching the Bible).” In practice, if one has “accepted Jesus Christ as his or her personal Savior” he/she is a member of the “church,” regardless of church affiliation or lack thereof.
Orthodox Christianity:
The Church is founded in God Himself, i.e. the communion of the Father, Son, and Holy Spirit.[3] The Church is the created expression of the life of the Holy Trinity which begins with the creation of the angels, continues in muted form in the Old Testament Church, and is perfected through the Incarnation, Death, and Resurrection of Jesus Christ, the God-Man, who is the only “Head” of the Church. A person or a community is part of the Church if he/she/it is grafted into the one Body of Christ and is guided by the apostolic Tradition of the Church through communion with the brotherhood of with bishops who guard and perpetuate this Tradition. The boundaries of the Church are visible. One must not only be in historical succession from the apostles but must also hold the apostolic faith they passed on.
From “The Orthodox Church” by Bishop Kallistos Ware:
“What then holds the Church together? Orthodox answer, the act of communion in the sacraments….Unity is not maintained from without by the authority of a Supreme Pontiff, but created within by the celebration of the Eucharist. The Church is not monarchical in structure, centered round a single hierarch; it is collegial, formed by the communion of many hierarchs with one another, and of each hierarch with the members of his flock. The act of communion therefore forms the criterion for membership of the Church. An individual ceases to be a member of the Church if he severs communion with his bishop; a bishop ceases to be a member of the Church if he severs communion with his fellow bishops.”
The Holy Scriptures on the Church:
Unity Expected in the Church
1 Cor. 16:1: “Now concerning the collection for the saints, as I have given orders to the churches of Galatia, so you must do also….”
Eph. 4:5: “…one Lord, one faith, one baptism….”
1 Tim. 2:8: “I desire therefore that the men pray everywhere, lifting up holy hands, without wrath and doubting….”
The Church as the Repository and Fullness of Truth:
1 Tim. 3:15: “…but if I am delayed, I write so that you may know how you ought to conduct yourself in the house of God, which is the church of the living God, the pillar and ground of the truth.”
Eph. 1:22-23: “And He put all things under His feet, and gave Him to be head over all things to the church, which is His body, the fullness of Him who fills all in all.”
Those who have departed (are separated) from the apostolic Church:
Acts 15:24: “Since we have heard that some who went out from us have troubled you with words, unsettling your souls, saying, “You must be circumcised and keep the law” —to whom we gave no such commandment….”
1 Jn. 2:19: “They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us.”
The Church’s Divine Power:
Eph. 3:10: “…to the intent that now the manifold wisdom of God might be made known by the church to the principalities and powers in the heavenly places….”
The Church is the Christ’s undivided Body:
1 Cor. 12:17: “Now you are the body of Christ, and members individually.”
Eph. 3:6: “that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ through the gospel….”
Eph. 4:4-6: “There is one body and one Spirit, just as you were called in one hope of your calling; one Lord, one faith, one baptism; one God and Father of all, who is above all, and through all, and in youall.”
Col. 1:24: “I now rejoice in my sufferings for you, and fill up in my flesh what is lacking in the afflictions of Christ, for the sake of His body, which is the church….”
Acts 9:4: “Then he fell to the ground, and heard a voice saying to him, ‘Saul, Saul, why are you persecuting Me?’”
Early Church Fathers on the Church:
St. Clement of Rome (96AD):
The Apostles received the gospel for us from the Lord Jesus Christ: Jesus the Christ was sent from God. Thus Christ is from God, the Apostles from Christ: in both cases the process was orderly, and derived from the will of God….They [the Apostles] preached in country and town, and appointed their first fruits, after testing them by the Spirit, to be bishops and deacons of those who were going to believe. And this was no novelty….Our Apostles also knew, through our Lord Jesus Christ, that there would be strife on the question of the bishop’s office. Therefore…they appointed the aforesaid persons and later made further provision that if they should fall asleep, other tested men should succeed to their ministry. (First Epistle to the Corinthians, xlii-xliv)
Tertullian (197AD):
The apostles, then, in like manner founded churches in every city, from which all the other churches—one after another—borrowed the tradition of the faith and the seeds of doctrine. And they are every day borrowing them, that they may become churches. Indeed, it is only on this account that they will be able to deem themselves apostolic—as being the offspring of apostle churches….Therefore the churches, although they are so many and so great, comprise but the one primitive church of the apostles—from which they all [spring].
We hold communion with the apostolic churches because our doctrine is in no respect different than theirs. This is our witness of truth. (3.252, 253).
St. Irenaeus of Lyons (202AD):
By “knowledge of the truth” we mean: the teaching of the Apostles; the order of the Church as established from the earliest times throughout the world: the distinctive stamp of the Body of Christ, preserved through the Episcopal succession: for to the bishops the Apostles committed the care of the church which is in each place, which has come down to our own time, safeguarded without any written documents…. (Against Heresies, iv. xxxiii. 8)
Origen (225AD):
We cling to the standard of the heavenly church of Jesus Christ according to the succession of the apostles.
St. Cyprian of Carthage:
“The episcopate is one….The Church is one….So also the Church, flooded with the light of the Lord, extends her rays over all the globe: yet it is one light which is diffused everywhere and the unity of the body is not broken up.” (On the Unity of the Catholic Church)
“This sacrament of unity [the Church], this bond of peace inseparable and indivisible, is indicated when in the Gospel the robe of the Lord Jesus Christ was not divided at all or rent, but they cast lots for the raiment…so the raiment was received whole and the robe was taken unspoilt and undivided.”
“…and the Church is made up of the people united to their priest, the flock cleaving to its shepherd. Hence you should know that the bishop is in the Church, and the Church in the bishop, and that if anyone is not with the bishop he is not in the Church…the Church is catholic and one, and may not be sundered or divided but should assuredly be kept together and united by the glue which is the mutual adherence of the priest.”
“Some of our colleagues, by a curious presumption, are led to suppose that those who have been dipped [baptized] among the heretics ought not to be baptized when they join us; because, they say, there is ‘one baptism’. Yes, but that one baptism is in the Catholic Church. And if there is one Church, there can be no baptism outside it….Our assertion is that those who come to us from heresy are baptized by us, not re-baptized.”