After Jesus' church was established on the first pentecost following His resurrection in A.D. 32, it underwent two things so amazing as to make one stop and consider it in awe. First, it faced the most massive and cruel persecution imaginable for approximately 3 centuries. This persecution was by the Jews as well as the Roman Empire. Christians were crucified, thrown to the lions and forced to fight to the death in the arenas. They were tortured, their eyes gouged out, maimed, plus many other things almost beyond the comprehension of the civilized world today.
Secondly, it's growth, dedication and stedfastness (for the most part) were as astounding as was it's persecution. Most of the martyrs were so by choice. Normally they were given the choice of renouncing Jesus Christ, or suffer a torturous death. These Christians were teaching the Gospel of Christ to others, knowing full well that they would probably die for it. Yet, their strength and faithfulness was so great, that they attracted many others to the cause. No doubt, this was a period of glory for His church that would never be exceeded.
But, even while this period of greatness was being made history, apostacy was beginning to show it's ugly head. In many places, congregational leadership (organization) was undergoing unauthorized change. Most of us are aware of this apostacy and how it came about, so we will by-pass that particular subject. But as time passed, a Pope was installed over the entire church early in the 6th century. Bishops were placed over five areas (bishoprics), Jerusalem, Antioch, Rome, Alexandria, and Constantinople. There was considerable disunity among the hierchy for some lengthy period, but when the Pope of Rome declared that the church (In Rome) as well as he, himself was in charge of the entire church, a split occurred with the Bishop of Constantinople declaring himself as head of the church. Thus the two "catholic" churches came into being. The one in Rome being known as the Roman Catholic Church while the one in Constantinople became known as the Eastern Orthodox, or Greek Orthodox Church.
I am enclosing some links for your reading to become better acquainted with the Eastern Orthodox Church. Although, their organization as well as their worship and doctrine are deeply embedded in apostacy, it is interesting to note the things that they did not do such as adding instruments of music into worship as did the church in Rome. Nor is their history as filled with infamous (criminal) activities as is the Roman church.