Nestorius developed an innovative theory on the person of Christ: he concluded that it is better to speak of Christ as two persons rather than one. One person was the man, and the other was the divine Logos, eternal and unbegotten. He proposed to speak of Mary as the Christotokos- mother of Christ and not the theotokos- mother of God. Yet every baptized Christian knew by heart, both from scripture and the liturgy that Mary had given birth to the eternal Son of God. It had been passed on from the earliest liturgical traditions that she, the mother of the God-man, was thus the mother of God incarnate, born as one person-truly human, truly divine. Nestorius had replaced the apostolic tradition with innovation. (The Rebirth of Orthodoxy, Thomas C. Oden, 2002)
While we tend not to get very alarmed in our day and age about new teachings that are presented from contemporary theological scholarship, the Vincentian canon does not favor innovations at all. In the case of Nestorius, we find a very bright and gifted teacher and scholar, and it is even conceivable that his views would probably be supported by many a Christian today, yet the security of the ancients delivered us from this error. They knew that they had always believed that Jesus is fully God and fully man. This then meant that Mary was the Theotokos- the mother of God, and not just the mother of an anointed man. This is where Vincent’s canon comes into play; while one may be able to support Nestorius’ view from the right set of biblical passages, we could not overcome what was delivered in the content of liturgies and the creeds. Hence Nestorius' innovation did not square with those what had always been believed. The Vincentian canon kept the Nestorian error from winning the day, and left us with one God-man.