In continuing the Half-Mil Bible history, we encounter the beginning of the 2nd millennium A.D. Since before the Council of Nicaea, there were fundamental divides between the Eastern Roman Empire and the West. It is difficult to narrow these issues down as they were theological, cultural, political, and personal.
It is difficult for the modern post-Reformation Christian to realize the significance of this event. Before The Schism (pronounced "Ski-zim"), there was one church. From the beginning of Christianity, you were either in The Church or you were out of The Church. There were no "alternatives," only the teachers of orthodoxy (lower case "o") or heretics.
In the end of the 1st millenium A.D., Eastern groups had developed their own practices, and differed from the West on several traditions. In 1054 these divisions became so deep that Pope Leo IX excommunicated (kicked out) the Patriarch (like an Archbishop) of Constantinople. In essence, Leo IX declared that the leader of the Eastern church was beyond salvation, and out of line with the true teachings of the Church.
The Patriarch, Michael Cerularius, returned the favor, formally declaring the Eastern church to no longer be under the authority of the Pope. To this day, the East remains loyal to their own Patriarchs, Metropolitans, and other leaders. While much of their tradition remains similar to Catholicism, there are many unique attributes to their faith and practice.
Catholicism and Eastern Orthodoxy stood as the first denominations. They have confronted each other on the world scale, though they have occasionally attempted reunification. This division is a major event in church history, though it is often forgotten by many in the West.
FYI: I found my own search bar just under the banner of this website quite useful for accurate information on this topic.
(This is part of the Half-Mil Bible history, focusing on key events in Judeo-Christian history every 500 years. The first post was on the Exodus, the second post was on the establishment of the monarchy, the third post was on the return from exile, the fourth post was about Christ, and the fifth post was on the Council of Nicaea.)