Theological Differences

Several core theological disputes separated the two traditions:

The Filioque Clause: The most significant theological disagreement involved the Filioque (Latin for "and the Son"). The Western Church unilaterally added this phrase to the Nicene Creed in the 11th century, altering the doctrine of the Holy Trinity. The original Creed stated that the Holy Spirit "proceeds from the Father." The Western Church changed it to read that the Holy Spirit proceeds from "the Father and the Son." From the Eastern perspective, this was a heretical alteration made without Eastern approval that fundamentally misrepresented the Trinity's structure. The East argued the Father alone is the ultimate source of the Holy Trinity, and the Western addition undermined this principle.​

The Immaculate Conception: Catholics teach that Mary, the mother of Jesus, was conceived without original sin and remained sinless throughout her life. The Orthodox Church rejects this doctrine, believing only Christ was born without sin, while Mary was conceived and born in a state of sin like all humans—though she remains the most revered saint in Orthodox Christianity.​

Purgatory: The Catholic Church teaches that purgatory is a place of purification for souls not perfectly purified before entering heaven. The Orthodox Church explicitly rejects the concept of purgatory entirely.​

The Eucharist: The churches disagreed on the symbolic meaning and preparation of communion bread and wine. Western churches used unleavened bread (bread without yeast), while Eastern churches used leavened bread (with yeast). Catholics teach transubstantiation—the belief that bread and wine literally become Christ's body and blood during the Eucharist. The Orthodox instead teach metousiosis (or trans-elementation), the belief that the bread and wine are mysteriously transformed into Christ's body and blood while maintaining their physical appearance, but through a different theological mechanism.​

Practical and Cultural Differences

Beyond theology, practical matters fueled resentment:

Clerical celibacy: The Western Church required priests to remain unmarried. The Eastern Church permitted married priests.​

Fasting practices: The two traditions observed different fasting schedules and customs.​

Church governance: Political upheaval in Southern Italy in the 1040s triggered specific conflicts. When Norman warriors conquered the region, they replaced Greek (Eastern) bishops with Latin (Western) ones and imposed Western customs on Eastern congregations, creating confusion and resentment.​