Nestorianism - an heresy
Nestorianism is the doctrine that Christ exists as two persons, the man Jesus and the divine Son of God, or Logos, rather than as two natures (True God and True Man) of one divine person. The doctrine is identified with Nestorius (c. 386–c. 451), Archbishop of Constantinople. This view of Christ was condemned at the Council of Ephesus in 431, and the conflict over this view led to the Nestorian schism, separating the Assyrian Church of the East from the Byzantine Church.
Nestorianism originated in the Church in the 5th century out of an attempt to rationally explain and understand the incarnation of the divine Logos, the Second Person of the Holy Trinity as Jesus Christ. Nestorianism taught that the human and divine essences of Christ are separate and that there are two natures, the man Jesus and the divine Logos, united in Christ. In consequence, Nestorians rejected such terminology as "God suffered" or "God was crucified", because the humanity of Christ which suffered is separate from his divinity. Likewise, they rejected the term Theotokos (Giver of birth to God/Mother of God) as a title of the Virgin Mary, suggesting instead the title Khristotokos (Giver of birth to Christ/Mother of Christ), because in their view he took only his human nature from his mother, while the divine Logos was pre-existent and external, so calling Mary "Mother of God" was misleading and potentially wrong.
The Assyrian Church of the East refused to drop support for Nestorius or to denounce him as a heretic. That church has continued to be called "Nestorian" in the West, to distinguish it from other ancient Eastern churches. However, the Church of the East does not regard its doctrine as truly Nestorian: it teaches the view of Babai the Great - Christ has two qnome (essences) that are unmingled and eternally united in one parsopa (personality). According to some interpretations, the origin of this belief is mostly historical and linguistic: for example, the Greeks had two words for 'person', which translated poorly into Syriac, and the meanings of these terms were not even quite settled during Nestorius's lifetime.
There are about 170,000 Nestorians today, mostly living in Syria, Iraq and Iran.