The Council of Chalcedon was summoned in 451 to consider the christological question in light of the "one-nature" picture of Christ proposed by Eutyches which prevailed at the "Robber Synod" of Ephesus in 449. The Council of Chalcedon promulgated the Definition of the Union of the Divine and Human Natures in the Person of Christ, which proved to be the touchstone of orthodox Christology (Text in BCP, p. 864). In carefully balanced phrases the Definition establishes that "one and the same Son, our Lord Jesus Christ" is "at once truly God and truly man (Latin, vere deus, vere homo), homoousios (of one substance) "with the Father as regards his Godhead" (against Arianism) and "with us as regards his humanity" (against Apollinarianism), in two natures-"without confusion, without change" (against Eutychianism), "without separation, without division" (against Nestorianism). Mary is recognized as theotokos, the God-bearer. See Christology; see Homoousios; see Monophysitism.
Glossary definitions provided courtesy of Church Publishing Incorporated, New York, NY,(All Rights reserved) from "An Episcopal Dictionary of the Church, A User Friendly Reference for Episcopalians," Don S. Armentrout and Robert Boak Slocum, editors.