Christ is the main subject of ‘Summa Theologiae: Third Part’. Aquinas centres his thoughts on the unity of the divine and human in the person of Christ, based on classical Christology. Was the Incarnation appropriate, and was it necessary in order to save sinners from damnation? Ultimately Aquinas' main concern is with Christ's true humanity, including his birth, passion and resurrection, and the multiple symbolism of the cross. The Sacraments are presented as both symbols and causes. In the case of the Eucharist, Aquinas cites three different understandings of what happens to the bread and wine — coexistence, annihilation, and transubstantiation.