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In Christian theology, ecclesiology is the study of the Church, the origins of Christianity, its relationship to Jesus, its role in salvation, its polity, its discipline, its eschatology, and its leadership.
In its early history, one of the Church's primary ecclesiological issues had to do with the status of Gentile members in what had become the New Testament fulfilment of the essentially Jewish Old Testament church. It later contended with such questions as whether it was to be governed by a council of presbyters or a single bishop, how much authority the bishop of Rome had over other major bishops, the role of the Church in the world, whether salvation was possible outside of the institution of the Church, the relationship between the Church and the State, and questions of theology and liturgy and other issues. Ecclesiology may be used in the specific sense of a particular church or denomination's character, self-described or otherwise. This is the sense of the word in such phrases as Catholic ecclesiology, Protestant ecclesiology, and ecumenical ecclesiology.
The word ecclesiology was defined in the 19th century as the science of the building and decoration of church buildings and is still used in that sense in the context of architectural history.
The roots of the word ecclesiology come from the Greek ἐκκλησία, ekklēsia (Latin: ecclesia ) meaning "congregation, church" [notes 1] and -λογία, -logia , meaning "words", "knowledge", or "logic", a combining term used in the names of sciences or bodies of knowledge.
The similar word ecclesialogy first appeared in the quarterly journal The British Critic in 1837, in an article written by an anonymous contributor  who defined it thus:
We mean, then, by Ecclesialogy, a science which may treat of the proper construction and operations of the Church, or Communion, or Society of Christians; and which may regard men as they are members of that society, whether members of the Christian Church in the widest acceptation of the term, or members of some branch or communion of that Church, located in some separate kingdom, and governed according to its internal forms of constitution and discipline. 
However, in volume 4 of the Cambridge Camden Society's journal The Ecclesiologist, published in January 1845, that society (the CCS) claimed that they had invented the word ecclesiology: 
...as a general organ of Ecclesiology; that peculiar branch of science to which it seems scarcely too much to say, that this very magazine gave first its being and its name. 
The Ecclesiologist was first published in October 1841 and dealt with the study of the building and decoration of churches. It particularly encouraged the restoration of Anglican churches back to their supposed Gothic splendour and it was at the centre of the wave of Victorian restoration that spread across England and Wales in the second half of the 19th century. Its successor Ecclesiology Today is still, as of 2017 [update] , being published by The Ecclesiological Society (successor to the CCS, now a registered charity). 
The situation regarding the etymology has been summed up by Alister McGrath: "'Ecclesiology' is a term that has changed its meaning in recent theology. Formerly the science of the building and decoration of churches, promoted by the Cambridge Camden Society, the Ecclesiological Society and the journal The Ecclesiologist, ecclesiology now stands for the study of the nature of the Christian church." 
Catholic ecclesiology today has a plurality of models and views, as with all Catholic Theology since the acceptance of scholarly Biblical criticism that began in the early to mid 20th century. This shift is most clearly marked by the encyclical Divino afflante Spiritu in 1943. Avery Robert Cardinal Dulles, S.J. contributed greatly to the use of models in understanding ecclesiology. In his work Models of the Church, he defines five basic models of Church that have been prevalent throughout the history of the Catholic Church. These include models of the Church as institution, as mystical communion, as sacrament, as herald, and as servant. 
The ecclesiological model of Church as an Institution holds that the Catholic Church alone is the "one, holy, catholic and apostolic Church", and is the only Church of divine and apostolic origin led by the Pope. This view of the Church is dogmatically defined Catholic doctrine, and is therefore de fide . In this view, the Catholic Church— composed of all baptized, professing Catholics, both clergy and laity—is the unified, visible society founded by Christ himself, and its hierarchy derives its spiritual authority through the centuries, via apostolic succession of its bishops, most especially through the bishop of Rome (the Pope) whose successorship comes from St. Peter the Apostle, to whom Christ gave "the keys to the Kingdom of Heaven". Thus, the Popes, in the Catholic view, have a God-ordained universal jurisdiction over the whole Church on earth. The Catholic Church is considered Christ's mystical body, and the universal sacrament of salvation, whereby Christ enables human to receive sanctifying grace.
The model of Church as Mystical Communion draws on two major Biblical images, the first of the "Mystical Body of Christ" (as developed in Paul's Epistles) and the second of the "People of God." This image goes beyond the Aristotelian-Scholastic model of "Communitas Perfecta" held in previous centuries. This ecclesiological model draws upon sociology and articulations of two types of social relationships: a formally organized or structured society (Gesellschaft) and an informal or interpersonal community (Gemeinschaft). The Catholic theologian Arnold Rademacher maintained that the Church in its inner core is community (Gemeinschaft) and in its outer core society (Gesellschaft). Here, the interpersonal aspect of the Church is given primacy and that the structured Church is the result of a real community of believers. Similarly, Yves Congar argued that the ultimate reality of the Church is a fellowship of persons. This ecclesiology opens itself to ecumenism  and was the prevailing model used by the Second Vatican Council in its ecumenical efforts. The Council, using this model, recognized in its document Lumen gentium that the Body of Christ subsists in a visible society governed by the Successor of Peter and by the Bishops in communion with him, although many elements of sanctification and of truth are found outside its visible structure. 
From the Eastern Orthodox perspective, the Church is one, even though it is manifested in many places. Eastern Orthodox ecclesiology operates with a plurality in unity and a unity in plurality. For Eastern Orthodoxy there is no 'either / or' between the one and the many. No attempt is made to subordinate the many to the one (the Roman Catholic model), nor the one to the many (the Protestant model). In this view, it is both canonically and theologically correct to speak of the Church and the churches, and vice versa.  Historically, that ecclesiological concept was applied in practice as patriarchal pentarchy, embodied in ecclesiastical unity of five major patriarchal thrones (Rome, Constantinople, Alexandria, Antioch and Jerusalem). 
There is disagreement between the Ecumenical Patriarchate of Constantinople and the Patriarchate of Moscow on the question of separation between ecclesiological and theological primacy and separation of the different ecclesiological levels:
Historical development of the Church of the East outside the political borders of the Late Roman Empire and its eastern successor, the Byzantine Empire, resulted in the creation of its distinctive theological and ecclesiological traditions, regarding not only the questions of internal institutional and administrative organization of the Church, but also the questions of universal ecclesiastical order. 
Martin Luther argued that because the Catholic Church had "lost sight of the doctrine of grace", it had "lost its claim to be considered as the authentic Christian church". This argument was open to the counter-criticism from Catholics that he was thus guilty of schism and the heresy of Donatism, and in both cases therefore opposing central teachings of the early Church and most especially the Church father St. Augustine of Hippo.  It also challenged the Catholic doctrine that the Catholic Church was indefectible and infallible in its dogmatic teachings.
There is no single "Radical Reformation Ecclesiology". A variety of views is expressed among the various "Radical Reformation" participants.
A key "Radical Reformer" was Menno Simons, known as an "Anabaptist". He wrote:
They verily are not the true congregation of Christ who merely boast of his name. But they are the true congregation of Christ who are truly converted, who are born from above of God, who are of a regenerate mind by the operation of the Holy Spirit through the hearing of the divine Word, and have become the children of God, have entered into obedience to him, and live unblamably in his holy commandments, and according to his holy will with all their days, or from the moment of their call. 
This was in direct contrast to the hierarchical, sacramental ecclesiology that characterised the incumbent Roman Catholic tradition as well as the new Lutheran and other prominent Protestant movements of the Reformation.
Some other Radical Reformation ecclesiology holds that "the true church [is] in heaven, and no institution of any kind on earth merit[s] the name 'church of God.'" 
For historical Protestant ecclesiology, see
Apostolic succession is the method whereby the ministry of the Christian Church is held to be derived from the apostles by a continuous succession, which has usually been associated with a claim that the succession is through a series of bishops. Those of the Anglican, Church of the East, Eastern Orthodox, Hussite, Moravian, Old Catholic, Oriental Orthodox, Catholic and Scandinavian Lutheran traditions maintain that "a bishop cannot have regular or valid orders unless he has been consecrated in this apostolic succession". These traditions do not always consider the episcopal consecrations of all of the other traditions as valid.
An episcopal polity is a hierarchical form of church governance in which the chief local authorities are called bishops. It is the structure used by many of the major Christian Churches and denominations, such as the Catholic, Eastern Orthodox, Oriental Orthodox, Church of the East, Anglican, Lutheran and Methodist churches or denominations, and other churches founded independently from these lineages.
The Eastern Orthodox Church, also called the Orthodox Church, is the second-largest Christian church, with approximately 220 million baptized members. It operates as a communion of autocephalous churches, each governed by its bishops via local synods. The church has no central doctrinal or governmental authority analogous to the head of the Catholic Church—the pope—but the Ecumenical Patriarch of Constantinople is recognized by them as primus inter pares. As one of the oldest surviving religious institutions in the world, the Eastern Orthodox Church has played a prominent role in the history and culture of Eastern and Southeastern Europe. The Eastern Orthodox Church officially calls itself the Orthodox Catholic Church.
Ecumenism, also spelled oecumenism, is the concept and principle that Christians who belong to different Christian denominations should work together to develop closer relationships among their churches and promote Christian unity. The adjective ecumenical is thus applied to any interdenominational initiative that encourages greater cooperation and union among Christian denominations and churches.
The highest-ranking bishops in Eastern Orthodoxy, Oriental Orthodoxy, the Catholic Church, the Hussite Church, Church of the East, and some Independent Catholic Churches are termed patriarchs.
The term Greek Orthodox Church has three meanings. The broader meaning designates "the entire body of Orthodox (Chalcedonian) Christianity, sometimes also called 'Eastern Orthodox,' 'Greek Catholic,' or generally 'the Greek Church'". The narrower meaning designates "any of several independent churches within the worldwide communion of (Eastern) Orthodox Christianity that retain the use of the Greek language in formal ecclesiastical settings". The third is the Church of Greece, the Eastern Orthodox church operating within the modern borders of Greece.
In ecclesiology, the Christian Church is what different Christian denominations conceive of as being the true body of Christians or the original institution established by Jesus. "Christian Church" has also been used in academia as a synonym for Christianity, despite the fact that it is composed of multiple churches or denominations, many of which hold a doctrinal claim of being the "one true church", to the exclusion of the others.
A Christian denomination is a distinct religious body within Christianity that comprises all church congregations of the same kind, identifiable by traits such as a name, particular history, organization, leadership, theological doctrine, worship style and sometimes a founder. It is a secular and neutral term, generally used to denote any established Christian church. Unlike a cult or sect, a denomination is usually seen as part of the Christian religious mainstream. Most Christian denominations self-describe themselves as churches, whereas some newer ones tend to interchangeably use the terms churches, assemblies, fellowships, etc. Divisions between one group and another are defined by authority and doctrine; issues such as the nature of Jesus, the authority of apostolic succession, biblical hermeneutics, theology, ecclesiology, eschatology, and papal primacy may separate one denomination from another. Groups of denominations—often sharing broadly similar beliefs, practices, and historical ties—are sometimes known as "branches of Christianity". These branches differ in many ways, especially through differences in practices and belief.
The East–West Schism, also known as the Great Schism or Schism of 1054, is the ongoing break of communion between the Roman Catholic and Eastern Orthodox churches since 1054. It is estimated that, immediately after the schism occurred, a slim majority of Christians worldwide were Eastern Christians; most of the rest were Western Christians. The schism was the culmination of theological and political differences between Eastern and Western Christianity that had developed during the preceding centuries.
Papal primacy, also known as the primacy of the bishop of Rome, is a Roman Catholic ecclesiological doctrine concerning the respect and authority that is due to the pope from other bishops and their episcopal sees. The doctrine is accepted at a fundamental level by both the Catholic Church and Eastern Orthodox Church, though the two disagree on the nature of primacy.
The term Evangelical Catholic is used in Lutheranism, alongside the term Augsburg Catholic, with those calling themselves Evangelical Catholic Lutherans or Lutherans of Evangelical Catholic churchmanship stressing the catholicity of historic Lutheranism in liturgy, beliefs, practices, and doctrines. Evangelical Catholics teach that Lutheranism at its core "is deeply and fundamentally catholic". The majority of Evangelical Catholic Lutheran clergy and parishes are members of mainstream Lutheran denominations.
Catholicity is a concept pertaining to beliefs and practices that are widely accepted by numerous Christian denominations, most notably by those Christian denominations that describe themselves as catholic in accordance with the Four Marks of the Church, as expressed in the Nicene Creed formulated at the First Council of Constantinople in 381: "[I believe] in one, holy, catholic, and apostolic Church."
Papal supremacy is the doctrine of the Catholic Church that the Pope, by reason of his office as Vicar of Christ, the visible source and foundation of the unity both of the bishops and of the whole company of the faithful, and as pastor of the entire Catholic Church, has full, supreme, and universal power over the whole church, a power which he can always exercise unhindered: that, in brief, "the Pope enjoys, by divine institution, supreme, full, immediate, and universal power in the care of souls."
The Four Marks of the Church, also known as the Attributes of the Church, describes four distinctive adjectives of traditional Christian ecclesiology as expressed in the Niceno-Constantinopolitan Creed completed at the First Council of Constantinople in AD 381: "[We believe] in one, holy, catholic, and apostolic Church."
The church invisible, invisible church, mystical church or church mystical, is a Christian theological concept of an "invisible" Christian Church of the elect who are known only to God, in contrast to the "visible church"—that is, the institutional body on earth which preaches the gospel and administers the sacraments. Every member of the invisible church is "saved", while the visible church contains all individuals who are saved though also having some who are "unsaved". According to this view, Bible passages such as Matthew 7:21–27, Matthew 13:24–30, and Matthew 24:29–51 speak about this distinction.
The Catholic Church and the Eastern Orthodox Church have been in a state of official schism from one another, with a few short-lived reunifications since the East–West Schism of 1054. That original schism was exacerbated by historical and language differences, and the ensuing theological differences between the Western and Eastern churches.
The expression "one true church" refers to an ecclesiological position asserting that Jesus gave his authority in the Great Commission solely to a particular visible Christian institutional church—what is commonly called a denomination. This view is maintained by the Catholic Church, the Eastern Orthodox Church, the Oriental Orthodox communion, the Assyrian Church of the East, the Ancient Church of the East, the Christian Churches/Churches of Christ, the Churches of Christ, and the Lutheran Churches, as well as certain Baptists. Each of them maintains that their own specific institutional church (denomination) exclusively represents the one and only original church. The claim to the title of the "one true church" relates to the first of the Four Marks of the Church mentioned in the Nicene Creed: "one, holy, catholic, and apostolic church". As such, it also relates to claims of both catholicity and apostolic succession: asserting inheritance of the spiritual, ecclesiastical and sacramental authority and responsibility that Jesus Christ gave to the apostles.
Paul David Loup Avis is an English Anglican priest, theologian, and ecumenist. He was General Secretary of the Church of England's Council for Christian Unity from 1998 to 2011, theological consultant to the Anglican Communion Office, London, from 2011 to 2012, and Canon Theologian of Exeter Cathedral from 2008 to 2013. He was honorary professor in the Department of Theology and Religion at Durham University 2017-2021 and Honorary Professor in the School of Divinity, University of Edinburgh 2022-. At the University of Exeter he was visiting professor of theology from 2009 to 2017 and is currently honorary research fellow. He is Director of the Centre for the Study of the Christian Church which organises occasional conferences and is linked to the journal Ecclesiology, published by Brill, of which he is Editor-in-Chief. He is the editor of the series Anglican-Episcopal Theology and History, also published by Brill. Avis was also a chaplain to Queen Elizabeth II, 2008–2017.
Several differences exist within the organizational structures and governance of the Catholic Church and the Eastern Orthodox Church. These are distinguished from theological differences which are differences in dogma and doctrine. A number of disagreements over matters of Ecclesiology developed slowly between the Western and Eastern wings of the State church of the Roman Empire centered upon the cities of Rome and New Rome/Constantinople respectively. The disputes were a major factor in the formal East-West Schism between Pope Leo IX and Patriarch Michael I in 1054 and are largely still unresolved between the churches today.
Catholic ecclesiology is the theological study of the Catholic Church, its nature and organization, as described in revelation or in philosophy. Such study shows a progressive development over time. Here the focus is on the time leading into and since the Second Vatican Council (1962–1965).