We celebrate Christ Jesus present in the Eucharist with a special feast, separate from the Institution of the Eucharist on Holy Thursday. This feast of Corpus Christi is our declaration that the presence of Jesus in the Eucharist is itself worthy of praise. The Eucharist not only commemorates the sacrifice of Jesus’ body and blood on the Cross, but the real, true, substantial Presence of Jesus in the Eucharistic species. After the words of the Eucharistic Prayer, we believe the bread and wine that were offered are Transubstantiated into the Body and Blood of Jesus. The Eucharist we share at the Altar is Jesus.
Transubstantiation is a big word, and a big concept, but it relies on relatively simple philosophical ideas: substance and accidents. In philosophy, substance is the ‘what’—the underlying thing itself (bread), and accidents are what we can say about that thing (color, texture, odor, taste, shape, temperature, mass, etc.). Transubstantiation means the underlying thing changes, but the accidents of bread and wine remain. Before, it is bread and wine; after, it is Jesus’ Body and Blood. The Host looks like bread before and after, smells like bread, tastes like bread, but after the Consecration it no longer is bread; it is Jesus. The Precious Blood of Jesus in the Eucharist is just as intoxicating as the wine it was before Transubstantiation, and consuming a large quantity of the Eucharist can fill you up just like normal food. Our senses are too limited to tell the difference; we can only know this by faith and Sacred Scripture.
We rely on what Jesus said, and St. Paul repeated in his First Letter to the Corinthians: “I received from the Lord what I also handed on to you, that the Lord Jesus, on the night he was handed over, took bread, and, after he had given thanks, broke it and said, "This is my body that is for you. Do this in remembrance of me." In the same way also the cup, after supper, saying, "This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me." We say and do what Jesus taught us to say and do, and we believe that we receive the same gift He gave in that first Eucharist.
Throughout the centuries, many Eucharistic miracles have reinforced our belief in the Real Presence of Jesus in the Eucharist. One miracle, in particular, was believed to have inspired our Feast of Corpus Christi. In 1263, the consecrated Host bled on the corporal, the small white cloth on the center of the altar. That bloodstained corporal is now displayed in the Cathedral of Orvieto, Italy, and reportedly inspired Pope Urban IV to institute the universal Feast of Corpus Christi in 1264. Another miracle, this one in Lanciano, Italy in the 8th Century, involved the Eucharist taking on the accidents of normal human flesh and blood. The Host became like human cardiac tissue, and the Precious Blood in the Chalice like normal blood (type AB, to be specific). Both of these miracles have been preserved, are still observable today, and have been tested repeatedly with modern scientific methods.
These are extraordinary reminders to us that what we celebrate in every Eucharist is the real, true, substantial Presence of Jesus Christ. In Holy Communion we receive Jesus; the Eucharist is His Body and Blood, shared with us as a foretaste and promise of the perfect union we will share with Jesus in heaven. We become what we receive, and we too are invisibly changed when we receive the Eucharist. So let us rejoice in so great a gift, and protect and give reverence and adoration to the Presence of Jesus in this and every Eucharist.