The union of the human and divine natures in the one divine person of Christ. At the Council of Chalcedon (A.D. 451) the Church declared that the two natures of Christ are joined "in one person and one hypostasis" (Denzinger 302), where hypostasis means one substance. It was used to answer the Nestorian error of a merely accidental union of the two natures in Christ. The phrase "hypostatic union" was adopted a century later, at the fifth general council at Constantinople (A.D. 533). It is an adequate expression of Catholic doctrine about Jesus Christ that in him are two perfect natures, divine and human; that the divine person takes to himself, includes in his person a human nature; that the incarnate Son of God is an individual, complete substance; and that the union of the two natures is real (against Arius), no mere indwelling of God in a man (against Nestorius), with a rational soul (against Apollinaris), and the divinity remains unchanged (against Eutyches).