Michael I Cerularius (c. 1000 – 1059), also known as Michael Keroularios or Patriarch Michael I, was the Patriarch of Constantinople from 1043 to 1059.
Born in Constantinople, Patriarch Michael I Cerularius is noted for disputing with Pope Leo IX over church practices in respect of which the Roman Church differed from Constantinople, especially the use of unleavened bread in the Eucharist.
Pope Leo IX sent a letter to the Patriarch in 1054, that cited a large portion of the Donation of Constantine believing it genuine. 
Leo IX assured the Patriarch that the donation was completely genuine, not a fable or old wives' tale, so only the apostolic successor to Peter possessed that primacy and was the rightful head of all the Church.
This letter of Pope Leo IX was addressed both to Michael Cærularius, Patriarch of Constantinople, and Leo of Ohrid, Archbishop of Bulgaria, and was in response to a letter sent by Leo, Metropolitan of Achrida to John, Bishop of Trani (in Apulia), that categorically attacked the customs of the Latin Church that differed from those of the Greeks. Especially criticized were the Roman traditions of fasting on the Saturday Sabbath and consecration of unleavened bread. Leo IX in his letter accused Constantinople of historically being the source of heresy and claimed in emphatic terms the primacy of the Bishop of Rome over even the Patriarch of Constantinople, who would have none of it.
In 1054, it can be argued
that the Patriarch's letter to Pope Leo IX initiated the events which followed because it claimed the title "ecumenical patriarch" and addressed Pope Leo as "brother" rather than "father." Pope Leo IX sent Cardinal Humbert of Silva Candida on a legatine mission to treat with the Patriarch. Cerularius refused to meet with Cardinal Humbert and kept him waiting with no audience for months.
Thus, Cardinal Humbert of Silva Candida delivered a notice of excommunication against Patriarch Michael on July 16, 1054, despite the death of Pope Leo three months prior and thus the invalidity of the excommunication. Michael in turn excommunicated the cardinal and the Pope and subsequently removed the pope's name from the diptychs starting the East-West Schism.
This schism led to the end of the alliance between the Emperor and the Papacy, and caused later Popes to ally with the Normans against the Empire. Patriarch Michael closed the Latin churches in his area which, of course, exacerbated the schism. In 1965, those excommunications were rescinded by Pope Paul VI and Patriarch Athenagoras when they met in the Second Vatican Council. Although the excommunication delivered by Cardinal Humbert was invalid, this gesture represented a significant step towards restoring communion between Rome and Constantinople.
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