The Great Schism

The Great Schism of the 11th century divided what was simply understood as "Christianity" in the Byzantine Empire into two sects, Roman Catholisism and Eastern Orthodoxy. The Schism was a series of mostly minor disagreements between the two churches that forced the two to split. One major disagreement between the two religions was the insertion of the Filioque Clause in the Nicene Creed. The original clause said "We believe in the Holy Spirit ... who proceeds from the Father", while the ammended clause said "We believe in the Holy Spirit ... who proceeds from the Father and the Son", which was not accepted by Orthodox Christains. Other disputes between the two churches included which religion had juristiction over the Balkans, Southern Italy, and Sicily, weather the Pope was in higher standing than the other patriarchs, the use of unleavened bread for the Eucharist, and the seperation of church and government to name a few.
Partly because of the Filioque Clause , and partly because of the Roman Catholic Church's use of unleavened bread, Patriarch Michael Cerularius closed the Roman Catholic Churches that he had control over. In response to this, three delagates from the Roman Catholic Church went from Rome to Constantinople (the heart of the Eastern Orthodox Church) and in the middle of mass, placed a decleration of excommunication on the altar, which was understood as excommunication of the entire Greek (or Orthodox) church. After that, the two religions were indefinatley split, dispite numerous efforts to reconcile them.

Sources:
http://encarta.msn.com/encyclopedia_761576891/Schism_Great.html
http://en.wikipedia.org/wiki/East-West_Schism#Catalysts
http://www.orthodoxwiki.org/Great_Schism