Crucial questions for Christianity to clarify:
1. Is Christianity monotheistic?
2. How is Jesus Christ the savior of humanity?

First Council at Nicaea

• Christological issue: Is Christ God? – begotten or unbegotten
• Major opponents: Alexander/Athanasius vs. Arius
• Council called 325 AD by Constantine
• Important terms: logos, homoousios/homoiousios
• Resulting decision or document: Arius’ teaching condemned; Nicene Creed

First Council of Constantinople (2nd ecumenical council)

• Christological issue: Is Christ human and divine?
• Trinitarian issue: Is the Spirit God?
• Major opponents: Cappadocians vs. Semi-Arians (pneumatomachoi) and Apollinaris
• Council called 381 AD by Theodosius I
• Important terms: one ousia and three hypostases
• Resulting decision or document: Apollinaris’ teaching condemned

Council of Ephesus (3rd ecumenical council)

• Christological issue: Is Christ 2 persons?
• Major opponents: Cyril of Alexandria (died 444) vs. Nestorius, Bishop of Constantinople (deposed 436, died later)
• Council called 431 AD by Theodosius II
• Important terms: hypostasis, theotokos/christotokos (reality of incarnation)
• Resulting decision or document: Nestorius’ teaching condemned (Cyril was restored to see of Alexandria; communion between Alexandria and Antioch was restored through Formula of Reunion, 433)

Council of Chalcedon (4th ecumenical council)

• Christological issue: Does Christ have one nature or two?
• Major opponents: Flavian, bishop of Constantinople (successor of Proclus and Nestorius) vs. Dioscorus, bishop of Alexandria (successor of Cyril), who supported Eutyches
• Preliminary conflict: Robber Council 449 AD – Flavian condemned, Eutyches upheld, Leo’s Tome ignored
• Council called 451 AD at Chalcedon by Empress Pulcheria and Marcian
• Important terms: physis
• Resulting decision or document: Dioscorus deposed; letters of Cyril and Leo’s Tome accepted, and Chalcedonian Definition adopted (against both Eutyches; and Nestorius’ positions)