Christianity in the 11th century is marked primarily by the Great Schism of the Church, which formally divided the State church of the Roman Empire into Eastern (Greek) and Western (Latin) branches.
In 1054, following the death of the Patriarch of Rome Leo IX, papal legates (representatives of the Pope) from Rome traveled to Constantinople to deny Michael Cerularius, the reigning Patriarch of Constantinople, the title of Ecumenical Patriarch and to insist that he recognize the Church of Rome's claim to be the head and mother of the churches. Cerularius refused, resulting in the leader of the contingent from Rome excommunicating Cerularius and the legates in turn being excommunicated by Constantinople. Though this event, in and of itself, was relatively insignificant (and the authority of the legates in their actions was dubious) it ultimately marked the end of any pretense of a union between the eastern and western branches of the Church. Though efforts were made at reconciliation at various times, they remained divided, either claiming to be the true Christian Church.