One excommunication in 1054 split Christianity in two for good

In 1054, a Roman cardinal laid a bull of excommunication on the altar of Hagia Sophia, and the Patriarch of Constantinople excommunicated him right back days later. The immediate dispute was over authority, the wording of the creed, and clerical customs, but it capped centuries of growing estrangement between the Latin West and the Greek East. The breach produced the Roman Catholic and Eastern Orthodox churches as they exist today, and it has never been formally healed.

— Humbert of Mourmoutiers and Michael I Cerularius, East–West Schism — 1054