Ancient Church of the East in the context of "Church of the East"

Sacred tradition, also called holy tradition, Anno Domini tradition, or apostolic tradition, is a theological term used in Christian theology. According to this theological position, sacred Tradition and Scripture form one deposit, so sacred Tradition is a foundation of the doctrinal and spiritual authority of Christianity and of the Bible. Thus, the Bible must be interpreted within the context of sacred Tradition (and vice versa) and within the community of the denomination. The denominations that ascribe to this position are the Catholic, Eastern Orthodox, and Oriental Orthodox churches, and the Assyrian churches (the Ancient Church of the East and the Assyrian Church of the East).

The Anglican and Methodist churches regard tradition, reason, and experience as sources of authority but as subordinate to scripture – a position known as prima scriptura. That is in contrast to the Lutheran and Reformed traditions, which teach that the Bible alone is a sufficient/infallible basis for all Christian teaching – a position known as sola scriptura. In Lutheranism, tradition is subordinate to Scripture and is cherished for its role in the proclamation of the Gospel.

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, 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.

The concept of schism somewhat moderates the competing claims between some churches—one can potentially repair schism, since they are striving for the same goal. For example, the Catholic and Eastern Orthodox churches each regard the other as schismatic and at very least heterodox, if not heretical, yet both have held dialogues and even partaken in Councils in attempts to resolve the division that exists between them.

Eastern Christianity comprises Christian traditions and church families that originally developed during classical and late antiquity in the Eastern Mediterranean region or locations further east, south or north. The term does not describe a single communion or religious denomination. Eastern Christianity is a category distinguished from Western Christianity, which is composed of those Christian traditions and churches that originally developed further west.

Major Eastern Christian bodies include the Eastern Orthodox Church and the Oriental Orthodox Churches, along with those groups descended from the historic Church of the East (also called the Assyrian Church), as well as the Eastern Catholic Churches (which are in communion with Rome while maintaining Eastern liturgies), and the Eastern Protestant churches. Most Eastern churches do not normally refer to themselves as "Eastern", with the exception of the Assyrian Church of the East and its offshoot, the Ancient Church of the East.

Christianity, which originated in the Middle East during the 1st century AD, is a significant minority religion within the region, characterized by the diversity of its beliefs and traditions, compared to Christianity in other parts of the Old World. Today, Christians make up approximately 5% of the Middle Eastern population, down from 13-20% in the early 20th century. Cyprus is the only Christian majority country in the Middle East, with Christians forming between 76% and 78% of the country's total population, most of them adhering to Eastern Orthodox Christianity. Lebanon has the second highest proportion of Christians in the Middle East, around 40%, predominantly Maronites. After Lebanon, Egypt has the next largest proportion of Christians (predominantly Copts), at around 10% of its total population. Copts of Egypt, numbering around 10 million, constitute the single largest Christian community in the entire Middle East.

The Eastern Aramaic speaking Assyrians of northern Iraq, northeastern Syria, southeastern Turkey, and parts of Iran have suffered due to ethnic cleansing, religious discrimination, and persecution for many centuries. During the 20th century, the percentage of Christians in the Middle East fell mainly as a result of the late Ottoman genocides: the Armenian genocide, Greek genocide, and Assyrian genocide committed against them by the Ottoman Turks and their allies, leading many to flee and congregate in areas in northern Iraq, northeastern Syria, North America, and Western Europe. The great majority of Aramaic speaking Christians are followers of the Assyrian Church of the East, Chaldean Catholic Church, Syriac Orthodox Church, Ancient Church of the East, Assyrian Pentecostal Church and Assyrian Evangelical Church. In Iraq, the numbers of Christians has declined to between 300,000 and 500,000 (from 0.8 to 1.4 million before 2003 US invasion). Assyrian Christians were between 800,000 and 1.2 million before 2003. In 2014, the population of the Nineveh Plains in northern Iraq was scattered to Dohuk, Erbil and Jordan due to ISIS forcing the Assyrian community out of their historical homeland, but since the defeat of the Islamic State in 2017, Christians have slowly began returning.

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 Christ. "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.

For many Protestant Christians, the Christian Church has two components: the church visible, institutions in which "the Word of God purely preached and listened to, and the sacraments administered according to Christ's institution", as well as the church invisible—all "who are truly saved" (with these beings members of the visible church). In this understanding of the invisible church, "Christian Church" (or catholic Church) does not refer to a particular Christian denomination, but includes all individuals who have been saved. This is in contrast to the one true church applied to a specific concrete Christian institution, a Christian ecclesiological position maintained by the Catholic Church, the Lutheran Churches, the Eastern Orthodox Church, the Oriental Orthodox churches, Assyrian Church of the East, and the Ancient Church of the East. While holding this view, the Lutheran Churches teach that "there are indeed true Christians in other churches" as "other denominations also preach the Word of God, though mixed with error". The branch theory, which is maintained by some Anglicans, holds that those Churches that have preserved apostolic succession are part of the true Church, though the majority of Anglicanism has historically "followed the major continental Reformers in their doctrine of the true church, identifiable by the authentic ministry of word and sacrament, in their rejection of the jurisdiction of the pope, and in their alliance with the civil authority ('the magistrate')".