Nestorius was taught the disunity of Christ's human and divine natures. Under that theory, Mary was only the mother of Jesus. Jesus the man and Jesus as God were two distinct entities, versus one.
Nestorius tried to answer a question considered unsolved: "How can Jesus Christ, being part man, not be partially a sinner as well, since man is by definition a sinner since the Fall". To solve that he taught that Mary, the mother of Jesus gave birth to the incarnate Christ, not the divine Logos who existed before Mary and indeed before time itself. The Logos occupied the part of the human soul (the part of man that was stained by the Fall). But wouldn't the absence of a human soul make Jesus less human? Nestorius rejected this proposition, answering that, because the human soul was based on the archetype of the Logos, only to become polluted by the Fall, Jesus was "more" human for having the Logos and not "less". Consequently, Nestorius argued that the Virgin Mary should be called Christotokos, Greek for "Birth Giver of Christ", and not Theotokos, Greek for "Birth Giver of God".
Nestorius believed that no union between the human and divine were possible. If such a union of human and divine occurred, Nestorius believed that Christ could not truly be con-substantial with God and con-substantial with us because he would grow, mature, suffer and die (which Nestorius argued God cannot do) and also would possess the power of God that would separate him from being equal to humans.
Nestorius's opponents charged him with detaching Christ's divinity and humanity into two persons existing in one body, thereby denying the reality of the Incarnation.