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 (died ) 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 doctrine of Nestorius, Patriarch of Constantinople (appointed in 428), by which Christ is asserted to have had distinct human and divine persons."

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 (died 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.

After the condemnation, some supporters of Nestorius, who were followers of the Antiochian School and the School of Edessa, relocated to the Sasanian Empire, where they were affiliated with the local Assyrian community of the satrapy of Asuristan (Assyria), many who were followers of the Assyrian Church, known as the Church of the East, while others were Syriac Orthodox. During the period from 484 to 612, gradual development led to the creation of specific doctrinal views within the Church of the East. Evolution of those views was finalized by prominent East Syriac theologian Babai the Great (died 628) who was using the specific Syriac term () as a designation for dual (divine and human) substances within one prosopon (person) of Christ. Such views were officially adopted by the Church of the East at a council held in 612.

Opponents of such views in the West inaccurately labeled them as "Nestorian", leading to the practice of mislabeling the Church of the East as Nestorian, and indeed the Assyrian people themselves as "Nestorians". However, in modern religious studies it has been criticized as improper and misleading, even though Nestorius is officially venerated as a saint in the Assyrian Church of the East. As a consequence, both in scholarly literature and in the field of inter-denominational relations, the term Nestorian increasingly focuses on its primary meaning, the original teachings of Nestorius, rather than referring to the teachings of the far older-originating Assyrian Church of the East or its offshoot, the Chaldean Catholic Church.

History

Nestorianism was condemned as heresy at the Council of Ephesus (431). The Armenian Church rejected the Council of Chalcedon (451) because they believed Chalcedonian Definition was too similar to Nestorianism. The Persian Nestorian Church, on the other hand, supported the spread of Nestorianism in Persarmenia. The Armenian Church and other eastern churches saw the rise of Nestorianism as a threat to the independence of their Church. Peter the Iberian, a Georgian prince, also strongly opposed the Chalcedonian Creed. Thus, in 491, Catholicos Babken I of Armenia, along with the Albanian and Iberian bishops met in Vagharshapat and issued a condemnation of the Chalcedonian Definition.

Nestorians held that the Council of Chalcedon proved the orthodoxy of their faith and had started persecuting non-Chalcedonian or Miaphysite Syriac Christians during the reign of Peroz I. In response to pleas for assistance from the Syriac Church, Armenian prelates issued a letter addressed to Persian Christians reaffirming their condemnation of the Nestorianism as heresy.

Following the exodus to Persia, scholars expanded on the teachings of Nestorius and his mentors, particularly after the relocation of the School of Edessa to the city of Nisibis under Persian control in 489, where it became known as the School of Nisibis. Nestorian monasteries propagating the teachings of the Nisibis school flourished in 6th century Persarmenia.

Despite this initial Eastern expansion, the Nestorians' missionary success was eventually deterred. David J. Bosch observes, "By the end of the fourteenth century, however, the Nestorian and other churches—which at one time had dotted the landscape of all of Central and even parts of East Asia—were all but wiped out. Isolated pockets of Christianity survived only in India. The religious victors on the vast Central Asian mission field of the Nestorians were Islam and Buddhism".

Doctrine

Nestorianism is described as a radical form of dyophysitism that, according to Chalcedonians, differs from their version of dyophysitism on several points, mainly by opposition to the concept of hypostatic union. It can be seen as the antithesis to Eutychian Monophysitism, which emerged in reaction to Nestorianism after the conclusion of the Council of Ephesus. Where Nestorianism holds that Christ had two distinct natures, divine and human, Monophysitism holds that he had but a single nature, his human nature being absorbed into his divinity. A brief definition of Nestorian Christology can be given as: "Jesus Christ, who is not identical with the Son but personally united with the Son, who lives in him, is one hypostasis and one nature: human."Martin Lembke, lecture in the course "Meetings with the World's Religions", Centre for Theology and Religious Studies, Lund University, Spring Term 2010. This contrasts with Nestorius' own teaching that the Word, which is eternal, and the Flesh, which is not, came together in a hypostatic union, 'Jesus Christ', Jesus thus being both fully man and God, of two ousia () (essences) but of one (person). Both Nestorianism and Monophysitism were condemned as heretical at the Council of Chalcedon and the Second Council of Ephesus.

Nestorius developed his Christological views as an attempt to understand and explain rationally the incarnation of the divine Logos, the Second Person of the Holy Trinity as the man Jesus. He had studied at the School of Antioch where his mentor had been Theodore of Mopsuestia; Theodore and other Antioch theologians had long taught a literalist interpretation of the Bible and stressed the distinctiveness of the human and divine natures of Jesus. Nestorius took his Antiochene leanings with him when he was appointed Patriarch of Constantinople by Byzantine emperor Theodosius II in 428.

Nestorius's teachings sparked controversy when he publicly challenged the long-established title of Theotokos ('God-Bearer') for the Virgin Mary. He argued that this title, while orthodox per se, was inadequate to fully encompass the nature of the Incarnation, as it singled out Christ's divinity with no reference to His humanity. To address this perceived limitation, he proposed the alternative title of Christotokos ('Christ-Bearer') as a more appropriate designation for Mary.

He also advanced the image of Jesus as a warrior-king and rescuer of Israel over the traditional image of the .

Nestorius' opponents found his teaching too close to the heresy of adoptionism – the idea that Christ had been born a man who had later been "adopted" as God's son. Nestorius was especially criticized by Cyril, Patriarch of Alexandria, who argued that Nestorius's teachings undermined the unity of Christ's divine and human natures at the Incarnation. Some of Nestorius's opponents argued that he put too much emphasis on the human nature of Christ, and others debated that the difference that Nestorius implied between the human nature and the divine nature created a fracture in the singularity of Christ, thus creating two Christ figures. Nestorius himself always insisted that his views were orthodox, though they were deemed heretical at the Council of Ephesus in 431, leading to the Nestorian Schism, when churches supportive of Nestorius and the rest of the Christian Church separated. However, this formulation was never adopted by all churches termed 'Nestorian'. Indeed, the modern Assyrian Church of the East, which reveres Nestorius, does not fully subscribe to Nestorian doctrine, though it does not employ the title Theotokos.

Intentions As written in his Bazaar of Heracleides, Nestorius went on to qualify his beliefs, including justifying his position, seemingly disconnecting him from the heresy of which bears his name. He would state that he never taught two persons or separated the Savior into two, affirming two natures in one person, that Mary is the mother of God; that Christ is God and Man. And, he "was born in flesh Christ, who is God above all." As John McGuckin says, "For Nestorius, a nature without a hypostasis was a 'phantom.' He used the word to mean the concrete particularity of the nature. His critics, however, read 'hypostasis' as 'Person,' and thus accused him of teaching two Persons when he was actually trying to teach two concrete realities."

Nestorius would live long enough to read and happily agree with the Tome of Leo proclaiming him 'guiltless' and 'truthful' as he articulated the teachings he was trying to express in dyophysitism.

As scholars note, Nestorius was trying to safeguard the distinctions of the two natures against confusion and mixture. For him, a nature without its own hypostasis (or concrete existence) was not a real nature at all. Nestorius defines hypostasis as the concrete reality of the nature, the 'that which' (id quod) of the essence. It is the individual instance which possesses the properties of the nature.... This is why he insists on two hypostases: to ensure the humanity is not swallowed up.