The moment
On 16 July 1054 Cardinal Humbert, a papal legate, laid a bull excommunicating Patriarch Michael Cerularius on the altar of Hagia Sophia, and the patriarch's synod answered by excommunicating the legates. Pope Leo IX had died in April, before the bull was laid. Many historians see the split as a gradual process, with the crusaders' sack of Constantinople in 1204, rather than 1054, as the point at which the breach became final.