The Eastern Orthodox Church. Well, I guess the place to start is, what is Orthodoxy? The early church (first few centuries) would refer to their tradition and teaching as catholic, apostolic, and orthodox. Catholic meaning universal. Apostolic meaning that which was passed down from the Apostles. The word orthodox comes from the Greek words orthos (right, straight, true) and doxa (belief, also close to the Greek word for thinking), so it means right belief or right thinking.
Why did early Christians feel the need to tack on these words? Why would some refer to themselves as catholic Christians? Isn’t Catholicism a medieval development? The first time in history that the word catholic was used (according to texts) was by St Ignatius of Antioch, he was a student of St John the Apostle. During this time there was no official Bible and no formal statement of faith, at least written down. All they had were some letters written by Apostles and their tradition, teaching. This teaching/tradition was passed down from the Apostles to their successors and so on. When weird teachings surfaced that combined some Christian teaching with hellenic philosophy and pagan mysticism (such as Zoroastrianism) the Church had to stand on something. Why? because these heretical teachings were taught by “Christians” that, guess what, used some Scripture (which remember there wasn’t an official, formalized Bible yet) to support their claims, they were known as Gnostics. So those Christians who adhered to the teachings of the Church stood on the apostolic tradition of interpretation of the Scriptures (which was not only how to read them but which letters and gospel texts were included within that tradition), they were the catholic Christians: the one, universal (catholic), apostolic Church (as it said later in the Creed). This tradition is Tradition; from Tradition came the Bible and the Creed (symbol of faith). The Creed is a written and spoken statement or symbol of orthodoxy, of right belief. The Creed is recited every day, and sort of “regulates” how one speaks about God and the Church. For Orthodox Christians, orthodoxy (lower case “o”) is closely tied to orthopraxis, meaning right practice, belief and practice are nearly inseparable.
For the first thousand years, the Church was unified. It wasn’t until 1054 that the Church split resulting in Western and Eastern Churches--Roman Catholic and Eastern Orthodox, respectively. Some doctrinal changes were already happening before the split, but it wasn’t until after the split (known as the Great Schism) that many things developed that became known as Roman Catholicism. Unfortunately, Orthodoxy was mostly unheard of in the West for centuries. And because of distance and language the Churches grew further and further apart.