Several early Christian scholars pondered the mystery of how Christ could be both God and man. One approach required the rejection of the essential unity of the two natures.
Nestorianism is a Christological doctrine advanced by Nestorius, Patriarch of Constantinople from 428–431. It emphasizes the disunion between the human and divine natures of Jesus. Nestorius developed his Christological views as an attempt to rationally explain and understand the incarnation of the divine Logos, the Second Person of the Holy Trinity as the man Jesus Christ.
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."
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, Pope (Patriarch) of Alexandria, who argued
that Nestorius' teachings undermined the unity of Christ's divine and human natures at
the Incarnation. Nestorius himself always insisted that his views were orthodox, though
they were deemed heretical at the First Council of Ephesus in 431, leading to the Nestorian
Schism, when churches supportive of Nestorius broke away from the rest of the Christian Church.
--Wikipedia