On 16 July 1054, in the middle of the liturgy, a papal legate laid a document of excommunication on the altar of Hagia Sophia against the patriarch Michael Cerularius, who responded in kind. The scene is dramatic and the date is in every textbook. It is also close to meaningless as an explanation.
The legate, Humbert of Silva Candida, was acting for a pope who had died three months earlier, which by the ordinary rules of canon law ended his commission and has left canonists arguing about whether the sentence was valid at all. The excommunications were personal, aimed at named individuals rather than at churches. Neither side treated the episode as a final rupture, and communion between Greek and Latin Christians continued in various places for a long time afterwards.
The real estrangement was slow and had many strands: the Filioque added to the creed in the West, the developing Roman claim to universal jurisdiction, the rivalry over the churches of southern Italy and the Balkans, different practices about leavened and unleavened bread and about clerical marriage, and above all the fact that after the seventh century very few Latins read Greek and very few Greeks read Latin. Two halves of a church that could not read each other's theology drifted for centuries before anyone laid anything on an altar.