In the West, the Roman Empire fell in 476 AD with the Germanic tribes and the sack of Rome. The Eastern Roman Empire which was centered in Constantinople now Istanbul in Turkey continued for another thousand years even after the fall of the Western Empire. In the East, Emperor Justinius continued as Emperor and as head of the Church in the Byzantine Empire.
Here we can find the seeds for the division that came in 1054. The West used Latin while the East used Greek. The iconoclasm of this period and criticisms of the Church of Rome moved the pope to excommunicate the Patriarch of the Byzantine Empire, Michael Cerularius. The East denied the primacy of the Pope and the infallibility of the Church and called the Pope the king of heretics. In 1204 we had the fourth Crusade and the sack of Constantinople which made the break complete.
The Eastern Empire continued using their own liturgy and language and in 1453 the city fell to the Ottoman Empire and the Orthodox moved to Russia. They called themselves Orthodox for having kept the correct development of belief from the time of Christ. Greece, Romania, Albania and the other Orthodox countries are independent autonomous churches.
460 years later in 1517, we have the beginning of the Protestant Reformation. The protest against the Catholicism of that time. Luther and Calvin and others began the break from Roman Catholicism. Divisions continued among themselves. A few years later we had the break from Rome with the Anglican Church and Henry VIII as the head of a new church.
The teachings of Orthodoxy have the same 7 Sacraments, the veneration of the Saints, the bishop, priest, and deacon hierarchical order as the Roman Church. Priests are allowed to marry. They use the Julian Calendar instead of the Gregorian Calendar which means they have different days for the Feasts of Easter and Christmas.
Protestants have difficulty with the respect shown to Mary, the Sacramental system, teaching on Purgatory, and stress only the Scriptures. Anglicans are in many ways similar to Catholics and their priests are allowed to marry, however, they follow the Protestants in accepting only 39 books of the Old Testament.
There is a desire to find unity among many of the Christians. After the Second Vatican Council, there was a strong movement to find unity with the Orthodox. In 1965 the Holy See and the Ecumenical Patriarchate Athenagoras of Constantinople withdrew the excommunications that were made at the Great Schism of 1054.
St. John Paul II made efforts to increase the contact with the Orthodox. In 1978 each year on the feast of St Andrew, representatives of the two churches meet for talks. The desire for the unity of Christianity is a wish of Jesus which all accept but the desire will need encounter and dialogue, efforts to rid ourselves of prejudices, discrimination, and a large dose of humility to acknowledge where mistakes were made and continue to be made by all the different members of the body of Christ which we know is meant to be one.
