The popes of Rome, held up as God's direct representatives on earth, often came into conflict with the Byzantine emperors of Constantinople, who also viewed themselves in a similar spiritual capacity. Though technically both centers of Christianity, Rome and Constantinople gradually drifted apart after a number of disagreements – some minute issues of doctrine, others larger issues of political power. These disagreements would ultimately lead to the Great Schism between the Roman Catholic and Eastern Orthodox churches.
The major theological differences between the pope and the Byzantine emperor centered on the "filioque," a Latin phrase that the pope and the western church inserted into the Nicene Creed, altering its meaning. The phrase, meaning "and from the son," changes the Creed to state that the Holy Ghost originates not just from the father but also from the son, putting the father and the son on equal footing, something the Byzantine emperors in the East could not agree to.
Issues of Supremacy
The issue of the filioque, however, is just a small doctrinal issue that masks the greater conflict over political supremacy. The Byzantine emperors in Constantinople viewed the popes as simply the bishops of Rome, and saw that city as being on equal footing in terms of spiritual authority with Constantinople, Alexandria, Antioch and Jerusalem, a system known as the Pentarchy. The popes, however, rejected the Pentarchy, and saw themselves as having spiritual supremacy over the other four major centers of Christianity.
The East-West Schism
The issue of the filioque and the larger issue of supremacy within the Church would lead to a crumbling of relations between Constantinople and Rome that would culminate in the East-West Schism of 1054. At this point, cultural differences between the Latin West and the Greek East were substantial, but a formal division finally occurred when the two cities mutually excommunicated each other.
The Fourth Crusade
Though the East-West Schism was on the surface simply making official something that had long been true, the fact that the two churches had finally separated led to several serious political consequences, most notably the Fourth Crusade. In 1198, Pope Innocent III called for a Crusade against the Egyptians. Instead, the crusaders chose to attack the Byzantine capital of Constantinople and establish a Latin Empire within Byzantine territory.
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