2. historical separation of churches: the separation of the Roman Catholic and Eastern Orthodox churches in 1054, as a result of theological disagreementhttp://www.byzcath.org/Faith-and-Worship/East...
In practice, many commentators have referred to 1054 as the year in which the Great Schism began, but in reality many of the Eastern Churches remained in communion with Rome following the events of 1054, and dialogue between East and West did not cease. An event of a far greater weight in East/West relations was the unfortunate sack of Constantinople by the Latin mercenaries of the Fourth Crusade in 1204 [...] In the 1960s, Pope Paul VI and Ecumenical Patriarch Athenagoras I mutually rescinded the personal excommunications of 1054 in a highly symbolic gesture of goodwill.http://www.ewtn.com/library/HOMELIBR/EASTWEST.TXT
In 1054, Pope Leo IX sent an emissary, Cardinal Humbert, from Rome to Constantinople. The cardinal's visit with Patriarch Cerularios was meant
to be a mission of conciliation. [..] The cardinal excommunicated the patriarch who, in turn, excommunicated the cardinal. The main point of contention was the use of leavened bread during the celebration of Mass, according to MacMillan Publishing's <Encyclopedia of Religion.> While it is commonly accepted that the separation of Rome and Constantinople into two Christian Churches was the result of centuries of conflict, the event became known as the Great Schism of 1054.