During the first millennium of Christianity, two centers of gravity emerged—Rome and Constantinople. As the centuries passed, differences in theology developed and a rivalry emerged between the pope in Rome and the patriarch in Constantinople. The two camps differed on issues such as whether priests could marry, the makeup of Eucharistic bread, days of fasting, and whether the Holy Spirit proceeds from the Father and the Son, or just from the Father.
In 1043 a rigid, ambitious churchman named Michael Cerularius was named patriarch of Constantinople, and five years later in Rome a French bishop rose to the papacy under the name Leo IX. During those days, Norman armies overran southern Italy and replaced Eastern (Orthodox) bishops from Constantinople with Western (Catholic) bishops from Rome. The new bishops started changing forms of worship, and when Michael Cerularius heard it, he retaliated by closing Roman churches in the Eastern regions.
Leo sent three men to Constantinople to deal with the problem. They were led by Humbert, a pompous, tactless man, who arrived in the imperial city denouncing, decrying, berating, and condemning the Orthodox leaders. The legates were housed in the imperial palace, but the patriarch ignored them, refusing even to see them. On July 16, 1054, as afternoon prayers were about to begin at Constantinople’s Hagia Sophia, Humbert marched through the cathedral and deposited on the high altar a parchment reading Videat Deus et Judicet excommunicating Michael Cerularius. Humbert then tromped out, shook the dust off his feet, and left the city. Four days later, on July 20, 1054, at the same place, Cerularius responded in kind and excommunicated the pope and his followers. He was supported by the patriarchs of Alexandria, Antioch, and Jerusalem.
The Great Schism had finally occurred. From that point, each side typically looked at the other not as brothers but as enemies or as heathen who needed to be converted. Today the Western Church is represented largely by the Catholic and Protestant bodies, and the Eastern Church by the Greek and Russian Orthodox faiths.
Whether we live or die, it must be for the Lord. Alive or dead, we still belong to the Lord. This is because Christ died and rose to life, so that he would be the Lord of the dead and of the living. Why do you criticize other followers of the Lord? Why do you look down on them? The day is coming when God will judge all of us. (Romans 14:8-10)
Robert J. Morgan, On This Day : 265 Amazing and Inspiring Stories About Saints, Martyrs & Heroes, electronic ed. (Nashville: Thomas Nelson Publishers, 2000, c1997). July 20.