The Orthodox Church, also officially called the Orthodox Catholic Church and commonly referred to in English-speaking countries as the Eastern Orthodox Church, is the world's second largest Christian communion, estimated to number 300 million members.

It asserts that it is the One, Holy, Catholic and Apostolic Church established by Jesus Christ and his Apostles almost 2,000 years ago. The Church is composed of several self-governing ecclesial bodies, each geographically and nationally distinct but theologically unified. Each self-governing (or autocephalous) body, often but not always encompassing a nation, is shepherded by a synod of bishops whose duty, among other things, is to preserve and teach the Apostolic and patristic traditions and related church practices. As in the Roman Catholic Church, Oriental Orthodoxy and some other churches, Orthodox bishops trace their lineage back to the Apostles through the process of Apostolic Succession.

The Orthodox Church traces its development back through the Byzantine and Roman empires, to the earliest church established by St. Paul and the Apostles. It practices what it understands to be the original ancient traditions, believing in growth without change. In non-doctrinal matters the church had occasionally shared from local Greek, Slavic and Middle Eastern traditions, among others, in turn shaping the cultural development of these nations.

The goal of Orthodox Christians from baptism, when it is believed that they are sealed with the Holy Spirit, is to continually draw near to God throughout life. This process is called theosis or deification and is a spiritual pilgrimage in which each person strives to become more holy and more "Christ Like" within Jesus Christ.

The Biblical text used by the Orthodox includes the Greek Septuagint and the New Testament. It includes the seven Deuterocanonical Books which are generally rejected by Protestants and a small number of other books that are in neither Western canon. Orthodox Christians use the term "Anagignoskomena" (a Greek word that means "readable", "worthy of reading") for the ten books that they accept but that are not in the Protestant 39-book Old Testament canon. They treat them on the same level as the others and use them in the Divine Liturgy. Orthodox Christians believe scripture was revealed by the Holy Spirit to its inspired human authors. They also use icons as a part of their personal and liturgical worship and prayer life. An Orthodox Christian will often have icons in their home and icons are a prominent feature in Orthodox churches. They are used in prayer and veneration of the saint or Biblical event they represent, but are not objects of worship themselves. The Orthodox Church maintains that this is not idolatry, nonetheless preferring them to three-dimensional statues. Icons depict Jesus Christ, the Virgin Mary, the saints and important Biblical events. They have been part of Orthodox Christianity since the beginning of the church.

Eastern Orthodox · Oriental Orthodox (Miaphysite) · Assyrian

Jehovah's Witness · Latter Day Saint · Unitarian · Christadelphian · Oneness Pentecostal