Council of Chalcedon
The Council of Chalcedon is the Fourth Ecumenical Council of the Catholic Church. In A.D. 451, this Council of Chalcedon decreed and determined, confirming previous Councils, that Jesus Christ is truly God, that Christ is also man, that Christ is One Person (not two), having Two Natures, the Divine Nature and the human Nature, and that the Deity and Humanity of Christ are not blurred, and are not united as one nature, divino-human, humano-divine. This Council thereby condemned the heresies of Monophysitism or Eutychianism.
In response to controversy and debate regarding which books are Scripture, the Council also affirmed and definitively listed 27 books of the New Testament and 46 books of the Old testament: 73 books of the Bible.
The Coptics and Apiscoliptics rejected this council, therefore becoming heretical in the eyes of the west. They have not returned to communion with Rome to this day.