Political, linguistic, theological, cultural and geographical differences between the Western and Eastern churches led to the East-West Schism of 1054. The proximate cause of the split was the mutual excommunication of the Patriarch of Constantinople and the Pope.
The roots of this split were very deep. First of all, the Western church was in Rome, while the Eastern church was in Constantinople. Furthermore, the Western church used Latin in its official documents and correspondence, while the Eastern church used Greek. This geographical and linguistic divide facilitated disagreements and made discussions more difficult when disputes occurred.
One such dispute regarded a clause in the Nicene Creed, a fundamental statement of belief in both the Eastern and Western churches. The Western church believed that the Holy spirit came from God the Father and the Son, and so inserted the word "filioque," which means "and the son," into the Nicene Creed. Leaders of the Eastern church were incensed at this change, causing a long and bitter theological argument. Partially in response to this argument, the pope attempted to assert his primacy over the Eastern church in 1054.
Also, the churches had different cultural practices. The Eastern church practiced clerical marriage, used leavened bread for the Eucharist, had different days of fasting and holidays and had a much more subordinate relationship to the political authority of the Byzantine emperor. The Western church practiced clerical celibacy, used unleavened bread and was relatively independent from the temporal authorities. The East-West Schism occurred because of all of these differences.