Sources: The Filioque controversy; Patriarch Michael Cerularius’s letter to the Pope (1054); the Acts of the Fourth Council of Constantinople (869 AD, Roman); the Hesychast controversy (Gregory Palamas vs. Barlaam, 1337–1351 AD).

The Great Schism of 1054 formally divides Eastern (Orthodox) and Western (Catholic) Christianity — though the split was a gradual process over centuries, not a single event. The immediate triggers: Cardinal Humbert of Silva Candida placed a bull of excommunication on the altar of Hagia Sophia against Patriarch Michael Cerularius; Cerularius excommunicated Humbert in turn. The underlying disagreements: (1) The Filioque — the Western church had added “and the Son” (filioque) to the Nicene Creed’s statement about the Spirit proceeding from the Father, without an ecumenical council’s authority; the East saw this as theological innovation and canonical violation; (2) Papal primacy — Rome claimed universal jurisdiction; Constantinople recognized a primacy of honor but not jurisdiction; (3) Jurisdictional disputes — particularly over the newly Christianized Bulgaria; (4) Liturgical differences — unleavened bread (azymes) in the West vs leavened bread (artos) in the East. Eastern Orthodox theology centers on theosis (deification — participation in the divine nature, 2 Peter 1:4) rather than Western juridical categories of sin and satisfaction. The Hesychast controversy (Palamas vs. Barlaam) produced the Eastern distinction between God’s essence (unknowable) and energies (participable) that remains central to Orthodox theology. The mutual excommunications were lifted in 1964 by Pope Paul VI and Patriarch Athenagoras.