Question and answer 80 of the Heidelberg Catechism contrast the biblical understanding of the Lord's Supper with the Roman Catholic view of the sacrament, highlighting the differences. One problem with the Roman Catholic understanding of the Eucharist, or the Lord's Supper, is the re-sacrifice of Christ in the Roman Catholic Mass. This denies the once-for-all character of the death of Jesus (Heb. 9:23–28). The second major concern with Rome's understanding of the Lord's Supper lies in its view of Christ's presence in the sacrament.
Roman Catholicism's teaching on Jesus' presence in the Lord's Supper is commonly known as transubstantiation. In transubstantiation, the bread and wine actually become the physical body and blood of our Savior when the priest consecrates the communion elements. Since the bread and wine evidence no outward change, Rome must use Aristotelian philosophy to defend its sacramental doctrine.
Aristotle distinguished the essence (or "substance") of an object from its accidents. The essence is that which makes something what it is. If an object's essential properties change, so does the object. For example, roundness is one essential property of a ball. If a ball were to lose its roundness, it would no longer be a ball. At the same time, objects have accidental, or nonessential, properties. An object's accidental properties can change without changing the object's essence. Color, for instance, is an accidental property of a ball. We could change a ball's color from blue to red, but we would still have a ball because color does not make a ball what it is as a ball.
In transubstantiation, the essence of the elements allegedly become our Lord's actual body and blood, but the accidents of bread and wine remain. The elements look, taste, touch, and feel like bread and wine even though they are, in essence, the physical body and blood of Christ. Therefore, transubstantiation ultimately locates Jesus' physical body and blood in more than one place at a time. His physical body ends up literally distributed all around the world every Lord's Day. This cannot be, for Jesus has a true human nature, and His physical body can be in only one place at a time. According to His humanity, Jesus cannot be in two different places at a time, and this is a conclusion we may draw from today's passage and many other texts.
Transubstantiation confuses the divine and human natures of Jesus, giving His human body the ability to be in more than one place at a time. This is an attribute that only the divine nature possesses. In confusing Christ's two natures, we get a Savior who is neither truly human nor truly divine but rather a heretofore unknown combination of Creator and creature. But a Jesus who is not truly human and truly divine cannot save us. What we believe about the Lord's Supper truly matters.