After Pentecost, the disciples and the 70 spread out and started churches in the cities they visited. They appointed leaders in every church (presbyters) and all the smaller cities fell under the leadership of a larger city (bishops). You can see this structure already in Acts 15 when the disciples came together for the Council of Jerusalem. James was the first bishop of Jerusalem and he presided over the meeting. So eventually 5 major cities formed the 5 “churches” if you will: Jerusalem, Antioch, Alexandria, Rome, and Constantinople. The bishops in those cities came to be known as patriarchs because they sat in leadership over all the surrounding area churches. This leadership was essential in the early church because there were many people running around claiming to be preaching about Christ but who were really teaching lies. There was no Bible until well into the 4th century and even then, the books weren’t set permanently for centuries after. So only by being part of one of the 5 churches could you know you were in the truth.
The Church in Rome was given a seat of honor and its patriarch was “first among equals” because it was the capital and because Peter and Paul had both sat in leadership there in the beginning. The Church functioned this way for 1000 years. Then in 1054, the patriarch of Rome determined that he should be the boss of all the churches, which the other 4 churches rejected because Christ is the one head of the Church, not any one man. The Church of Rome then broke from the other 4 churches, giving us 2 branches of Christianity, Roman Catholic and Eastern Orthodox. The RCC then went astray in many ways because of the lack of balance in leadership, adding new doctrines and practices along the way. This lead to the Protestant Reformation in 1592, from which we have all Protestant denominations.
The Orthodox Church, on the other hand, has not changed its doctrine or practice in 2000 years. They held to what they had been given by the apostles and those they had appointed after them. You can read about the early fathers of the church from the 100s and 200s and see the same exact worship services and customs that take place today. Of course, some things have changed, been given new meaning, etc, but nothing doctrinally or essential for salvation, just appearances. Today, it seems confusing because you see names like the Church of Greece or Russia or Jerusalem but these are just designations of different groups of Orthodox Christians. They may have some different customs or slightly different ways of doing things but they are all in communion with each other. We are all one Church, share one Lord, and partake of one cup (communion).
There is obviously much more in the details but that is a broad overview of who we are and where we come from. For me, once I realized what it is and that there is a direct line back what the apostles taught and that this had been preserved within the Orthodox faith through millennia, I decided I needed to be a part of that.