<P> In 1053 Leo of Ohrid, at the instigation, according to J.B. Bury, of Patriarch Michael Cerularius of Constantinople, wrote to Bishop John of Trani a letter, intended for all the Latin bishops, including the pope, in which he attacked Western practices such as using unleavened bread for the Eucharist, and fasting rules that differed from those in Constantinople, while Cerularius himself closed all Latin churches in Constantinople . </P> <P> In response, Leo IX wrote the letter In terra pax of 2 September 1053, addressed to Cerularius and Leo of Ohrid, in which he speaks at length of the privileges granted through Saint Peter to the see of Rome . In one of the 41 sections of his letter he also speaks of privileges granted by the emperors, quoting from the Donation of Constantine document, which he believed to be genuine (section 20). Some scholars say that this letter was never actually dispatched, but was set aside, and that the papal reply actually sent was the softer but still harsh letter Scripta tuae of January 1054 . </P> <P> The advance of the Norman conquest of southern Italy constituted a threat to the possessions of both the Byzantine Empire and the papacy, each of which sought the support of the other . Accordingly, conciliatory letters, the texts of which have not been preserved, were written to the pope by the emperor and Cerularius . In his January 1054 reply to the emperor, Quantas gratias, Leo IX asks for his assistance against the Normans and complains of what the pope saw as Caerularius's arrogance . In his reply to Caerularius, he upbraided the patriarch for trying to subject the patriarchs of Alexandria and Antioch to himself and for adopting the title of Ecumenical Patriarch, and insisted on the primacy of the see of Rome . </P> <P> These two letters were entrusted to a delegation of three legates headed by the undiplomatic Humbert of Silva Candida . They were given friendship and support by the emperor but were spurned by the patriarch . Finally, on 16 July 1054, three months after Pope Leo's death in April 1054 and nine months before the next pope took office, they laid on the altar of Hagia Sophia, which was prepared for celebration of the Divine Liturgy, a bull of excommunication of Cerularius and his supporters . At a synod held on 20 July 1054, Cerularius in turn excommunicated the legates . In reality, only Michael may have been excommunicated along with his then - living adherents . </P>

Why did the orthodox church split from the catholic church