⚠ Our source text has very little on him — Olson records only the mutual excommunication of 1054. He gives no life, no dates of birth or death, and nothing of his theology. We will not write it for him: on this thread he is a moment, not an idea. The break of 1054 needed a named person, and that is all. ⭐ What really needs reading is the two hundred years behind this break (see the two entries The Filioque Controversy and The Great Schism).
Below: first his own position, then the church’s verdict — kept separate. A rejected opinion keeps equal weight here.
His own position
On 16 July 1054 the papal legates laid a bull of excommunication on the altar of the cathedral of Hagia Sophia, excommunicating him; he at once called a synod and answered with an excommunication of his own. ⚠ For two hundred years before him, East and West had already been accusing each other over the filioque and over authority: in 809 a synod at Aachen in Germany affirmed the filioque as orthodox doctrine (Pope Leo III refused to write it into the text of the creed), and the East protested that the West had changed the creed without its consent. This answer of his was the last link in that chain.
The church’s verdict
⭐ The representatives of Pope Leo IX moved first. Olson describes how they entered the cathedral of Hagia Sophia, openly read out a declaration that excommunicated the patriarch of Constantinople and the bishops in fellowship with him who sat at the high altar, and then strode out of the cathedral. Cerularius then called a synod and answered with an excommunication of his own. ⚠ Olson gives only one sentence on the outcome, and we record it here: the rift has never healed.