Michael I Cerularius in the context of "Rome–Constantinople schism of 1054"

The East–West Schism, also known as the Great Schism or the Schism of 1054, is the break of communion between the Catholic Church and the Eastern Orthodox Church since 1054. A series of ecclesiastical differences, theological disputes and geopolitical tensions between the Greek East and Latin West preceded the formal split that occurred in 1054. Prominent among these were the procession of the Holy Spirit (Filioque), whether leavened or unleavened bread should be used in the Eucharist, iconoclasm, the coronation of Charlemagne as emperor of the Romans in 800, the pope's claim to universal jurisdiction, and the place of the See of Constantinople in relation to the pentarchy.

The first action that led to a formal schism occurred in 1053 when Patriarch Michael I Cerularius of Constantinople ordered the closure of all Latin churches in Constantinople. In 1054, the papal legate sent by Leo IX travelled to Constantinople in order, among other things, to deny Cerularius the title of "ecumenical patriarch" and insist that he recognize the pope's claim to be the head of all of the churches. The main purposes of the papal legation were to seek help from the Byzantine emperor, Constantine IX Monomachos, in view of the Norman conquest of southern Italy, and to respond to Leo of Ohrid's attacks on the use of unleavened bread and other Western customs, attacks that had the support of Cerularius. The historian Axel Bayer says that the legation was sent in response to two letters, one from the emperor seeking help to organize a joint military campaign by the eastern and western empires against the Normans, and the other from Cerularius. When the leader of the legation, Cardinal Humbert of Silva Candida, O.S.B., learned that Cerularius had refused to accept the demand, he excommunicated him, and in response Cerularius excommunicated Humbert and the other legates. According to Kallistos Ware, "Even after 1054 friendly relations between East and West continued. The two parts of Christendom were not yet conscious of a great gulf of separation between them ... The dispute remained something of which ordinary Christians in East and West were largely unaware".

Humbert of Silva Candida, O.S.B., also known as Humbert of Moyenmoutier (c. 1000 to 1015 – 5 May 1061) was a French Benedictine abbot and later cardinal. It was his act of excommunicating the Patriarch of Constantinople, Michael I Cerularius, in 1054 that is generally regarded as the precipitating event of the East–West Schism between the Roman Catholic Church and the Eastern Orthodox Church.

Abū 'l-Ḥasan al-Muḫtār Yuwānnīs ibn al-Ḥasan ibn ʿAbdūn ibn Saʿdūn ibn Buṭlān (Arabic: أبو الحسن المختار إيوانيس بن الحسن بن عبدون بن سعدون بن بطلان listen; c. 1001 to 1025 – 8 Šauwāl 458 AH or 2 September 1066), commonly known as Ibn Buṭlān (ابن بطلان listen), was an Arab physician and Nestorian Christian theologian. Born in Baghdad, the erstwhile capital city of the Abbasid Caliphate, he travelled throughout Mesopotamia, Syria, Egypt, and Anatolia, during which time he practiced medicine, studied, wrote, and engaged in intellectual debates—most famously the Battle of the Physicians with the Egyptian polymath Ibn Riḍwān. In 1054, he was in Constantinople, the capital city of the Byzantine Empire, where he witnessed first-hand the East–West Schism among Christendom, contributing a work to the discussions surrounding it for Michael I Cerularius, who was serving as the Patriarch of Constantinople. After his time in Constantinople, Ibn Buṭlān remained in the Byzantine Empire and eventually became a monk for the Greek Orthodox Patriarchate of Antioch amidst the end of the Macedonian Renaissance.

He is most renowned for his work Taqwīm aṣ-Ṣiḥḥa (تقويم الصحّة listen, lit. 'Tabular Register of Health'), a handbook on dietetics and hygiene. It was named for its intricate tables, similar to those found within a Taqwīm as-Sana (تقويم السنة, lit. 'tabular register of the year'), a type of astrological almanac. He was the first person to use these tables in a non-astrological work, creating a new scientific writing format that may be seen as the main influence for works like Taqwīm al-Abdān fī Tadbīr al-Insān by the Arab physician Ibn Ǧazla and Taqwīm al-Buldān by the Kurdish geographer and historian Abū 'l-Fidāʾ. Translations of Taqwīm aṣ-Ṣiḥḥa into Latin are preserved in many manuscripts from the early modern period, and are thought to illustrate the relationship between medieval Europe and the Arab world in the field of medicine. Despite increased European contact with Egypt and Syria through the Crusades and trade into the 16th century, there are no Latin translations of Arabic medical texts after Ibn Buṭlān's era.