While Eucharistic symbolism is well suited to helping us understand the effect that is proper to this Sacrament – the unity of the Mystical Body – still it does not indicate or explain what it is that makes this Sacrament different from all the others. For the constant teaching that the Catholic Church has passed on to her catechumens, the understanding of the Christian people, the doctrine defined by the Council of Trent, the very words that Christ used when He instituted the Most Holy Eucharist, all require us to profess that “the Eucharist is the flesh of Our Savior Jesus Christ which suffered for our sins and which the Father in His loving kindness raised again.” To these words of St. Ignatius, we may well add those which Theodore of Mopsuestia, who is a faithful witness to the faith of the Church on this point, addressed to the people: “The Lord did not say: This is symbol of my body, and this is a symbol of my blood, but rather: This is my body and my blood. He teaches us not to look to the nature of what lies before us and is perceived by the senses, because the giving of thanks and the words spoken over it have changed it into flesh and blood.”
-from Mysterium Fidei (1965)
