Patriarch Michael I

Michael I Cerularius
Ecumenical Patriarch of Constantinople
Michael I Cerularius
See	Patriarchate of Constantinople
Installed	1043
Term ended	21 January 1059
Predecessor	Alexius I Studites
Successor	Constantine III Lichoudas
Personal details
Birth name	Michael Keroularios
Born	c. 1000 Constantinople, Byzantine Empire
Died	21 January 1059 Constantinople, Byzantine Empire
Nationality	Byzantine
Denomination	Eastern Orthodoxy
Residence	Constantinople

Michael I Cerularius or Cærularius, also Hellenized as Keroularios (Greek: Μιχαήλ Α΄ Κηρουλάριος; c. 1000 – 21 January 1059 AD) was the Ecumenical Patriarch of Constantinople from 1043 to 1059 AD, most notable for his mutual excommunication with Pope Leo IX that led to the Great Schism.

Michael Cerularius was born in Constantinople around 1000 AD, being ordained into the Church from a young age. He 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. Notable disagreements were also exchanged over other theological and cultural issues, ranging from the issue of papal supremacy in the Church to the filioque clause and other disagreements between the Patriarchates.

Pope Leo IX sent a letter to the Patriarch in 1054, that cited a large portion of the Donation of Constantine believing it genuine.

Some scholars say that this letter of September 1053, the text of which is available in Migne, Patrologia Latina, vol. 143, coll. 744-769, was never actually despatched, but was set aside, and that the papal reply actually sent was the softer but still harsh letter Scripta tuae of January 1054.

Leo IX assured the Patriarch that the donation was completely genuine, not a fable or old wives' tale, arguing that only the apostolic successor to Peter possessed primacy in the Church.

This letter of Pope Leo IX was addressed both to Michael Cerularius, 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.