Prosopic union in the context of "Hypostasis (religion)"

Nestorius of Constantinople (/ˌnɛsˈtɔːriəs/; Ancient Greek: Νεστόριος; c. 386 – c. 451) was an early Christian prelate who served as Archbishop of Constantinople from 10 April 428 to 11 July 431. He was a Christian theologian from the Catechetical School of Antioch, and several of his teachings in the fields of Christology and Mariology were seen as controversial and heretical, causing major disputes. In 431, he was condemned and deposed from his see by the Council of Ephesus, presided over by his archrival Cyril of Alexandria, but the counter-council led by John I of Antioch vindicated him and deposed Cyril in return. Nestorius refrained from attending both of these councils and instead sought retirement from the Byzantine Emperor.

Nestorius himself used the title Theotokos and did not advocate for its ban. His teachings included cautious usage of Theotokos ("God-Bearer"), used for Mary, mother of Jesus, in order that Christ's human and divine natures not be confused, as he believed Christ was born according to his humanity and not his divinity, which indicated his preference for the concept of the prosopic union of two natures (divine and human) of Christ, over the concept of a hypostatic union. He proposed instead the title of Christotokos ("Christ-bearer"). This brought him into conflict with Cyril of Alexandria and other prominent diophysite and miaphysite churchmen of the time, who accused him of heresy.

Nestorianism is a term used in Christian theology and Church history to refer to several mutually related but doctrinally distinct sets of teachings that fall under the umbrella term Dyophysitism, such as two natures in Christ (human and Divine) or two persons in Christ (the Man and the Word). The extent to which those two definitions are actually distinct is also debatable. The first meaning of the term is related to the teachings of Christian theologian Nestorius (d. c. AD 451) as according to his immediate opponents at the Council of Ephesus and traditionally used by Miaphysites. The second meaning of the term relates to a set of later theological teachings that were traditionally labeled as Nestorian by Chalcedonians but differ in the teachings of Nestorius in origin, scope and terminology. Per the latter definition, the Oxford English Dictionary defines Nestorianism as:

The original definition of Nestorianism, as articulated by Nestorius himself, is preserved primarily in his surviving writings on topics such as Mariology and Christology. Although many of his works were lost or destroyed, others have been transmitted through his opponents or preserved in Church of the East libraries. Most notable among these is the Bazaar of Heracleides, composed during his exile following the Council of Chalcedon. The modern rediscovery of the Bazaar has prompted renewed scholarly interest in reconstructing Nestorius’s own theological positions, which appear to diverge in significant respects from the “two‐person” formulation of Christology attributed to him by both his contemporaries and later critics. His theology was influenced by teachings of Theodore of Mopsuestia (d. 428), the most prominent theologian of the Antiochian School. Nestorian Mariology prefers the title Christotokos, which encompasses the term Theotokos ('God-bearer') for Mary, thus emphasizing distinction between divine and human aspects of the Incarnation, and at the same time their unity in the person of Christ. Nestorian Christology promotes the concept of a prosopic union of two concrete realities (divine and human) in Jesus Christ, as opposed to the concept of a hypostatic union of two hypostases into one. The distinction is between 'two hypostases in one person' and 'two hypostases united into one hypostasis', respectively. Hypostasis is not seen as subject, but rather a nature existing in reality. This Christological position is viewed by the West as radical dyophysitism, and, according to Chalcedonian Christianity, differs from their dyophysitism, which was reaffirmed at the Council of Chalcedon in 451. Such teachings brought Nestorius into conflict with other prominent church leaders, most notably Cyril of Alexandria, who issued 12 anathemas against him in 430. Nestorius and his teachings were eventually condemned as heretical at the Council of Ephesus in 431, and again at the Council of Chalcedon in 451. His teachings were considered as heretical not only in Chalcedonian Christianity, but even more so in Oriental Orthodoxy. The Church of the East would affirm the orthodoxy of Nestorius, lining up with the tradition of the School of Antioch of its time.