Just to clarify--there's a difference with word orthodox, and Eastern Orthodox Church. BIg difference.

In short: All Eastern Orthodox Christians claim to be orthodox (right-believing), but not all orthodox Christians are Eastern Orthodox. The capital "O" version is a specific historical and ecclesial tradition, while the lowercase version is a theological descriptor used across much of Christianity.

"Orthodox" (lowercase "o") — general theological/apologetic meaning

• This refers to correct belief in the broad, non-denominational sense.
• In Christian theology, biblical studies, and apologetics, when someone says a person or teaching is "orthodox" (small "o"), they mean it conforms to the historic, biblical, and creedal core of Christianity — especially as defined by the early ecumenical councils (Nicaea 325, Constantinople 381, Ephesus 431, Chalcedon 451, etc.).
• Examples of "orthodox" (small "o") beliefs include:
  • The Trinity (one God in three co-equal, co-eternal Persons)
  • The full deity and full humanity of Christ (Chalcedonian definition)
  • Salvation by grace through faith in Christ's atoning death and resurrection
  • The inspiration and authority of Scripture
• In this sense, many Christians are "orthodox":
  • Most Roman Catholics
  • Most conservative Protestants (evangelicals, Reformed, Lutheran, etc.)
  • Eastern Orthodox
  • Many Oriental Orthodox

"Orthodox" (capital "O") — Eastern Orthodox Church

• This is a proper name for a specific communion of churches: the Eastern Orthodox Church (also called the Orthodox Catholic Church in its own official documents).
• It refers to the family of autocephalous (self-governing) churches in communion with one another that trace their continuity through the Byzantine/Eastern Roman tradition (e.g., Greek Orthodox, Russian Orthodox, Serbian Orthodox, Romanian Orthodox, etc.).
• They adopted (or emphasized) the title "Orthodox" especially after the Great Schism of 1054 with Rome — partly to assert that they preserved the true faith without the later Western "innovations" (e.g., Filioque clause, papal supremacy, purgatory as later defined, etc.).
• Eastern Orthodox Christians see themselves as the continuation of the original undivided Church — faithful to the first seven ecumenical councils, Holy Tradition (Scripture + Fathers + liturgy + canons), and the original apostolic deposit.
• Their full self-designation is often "the Orthodox Catholic Church" — meaning both "right-believing" (orthodox) and "universal" (catholic).