One reason for the Great Schism was Michael Cerularius' disagreement with the Catholic practice of not allowing clergy members to marry. Another reason was the Latin Church's addition of the term "filioque" to the Nicene Creed. A third reason occurred when Cerularius excommunicated bishops of Constantinople for using the term.
The Nicene-Constantinopolitan Creed originally referenced the Holy Spirit as proceeding only from the Father. However, the addition of the term "filoque" in the West implied that the Holy Spirit proceeds from both the Father and the Son. Michael Cerularius, who was patriarch of Constantinople at the time of the Great Schism, disagreed with this inclusion, prompting him to excommunicate any bishops who followed the Western ritual of using the term and close their churches. This action infuriated Pope Leo I in Rome.
Although Pope Leo insisted that Cerularius submit to his authority, Cerularius refused, and Pope Leo sent legates to talk to Cerularius. Pope Leo died before the discussion took place, and Cerularius refused to speak with Cardinal Humbert, the head of the legates, perceiving his demeanor to be rude. As a result, the legates put a bull of excommunication on the altar at Saint Sophia against Cerularius. Following this decision, the Church became divided into Eastern Orthodox and Roman Catholic churches.