Theology of transubstantiation
“Substance” here means what something is in itself. (For more on the philosophical concept, see Substance theory.) A hat’s shape is not the hat itself, nor is its color the hat, nor is its size, nor its softness to the touch, nor anything else about it perceptible to the senses. The hat itself (the “substance”) has the shape, the color, the size, the softness and the other appearances, but is distinct from them. While the appearances, which are referred to by the philosophical term accidents, are perceptible to the senses, the substance is not.
When at his Last Supper Jesus said: “This is my body”, what he held in his hands had all the appearances of bread: these “accidents” remained unchanged. However, the Roman Catholic Church believes that, in accordance with what Jesus said, the underlying reality was changed: the “substance” of the bread was converted to that of his body. In other words, it actually was his body, while all the appearances open to the senses or to scientific investigation were still those of bread, exactly as before. The Church holds that the same change of the substance of the bread and of the wine occurs at the consecration of the Eucharist.
The bread is changed into Jesus’ body, but, because Jesus, risen from the dead, is living, not only his body is present, but Jesus as a whole, body and blood, soul and divinity. The same holds for the wine changed into his blood.
In accordance with this belief that Christ is really, truly and substantially present under the remaining appearances of bread and wine, and continues to be present as long as those appearances remain, the Catholic Church preserves the consecrated elements, generally in a church tabernacle, for administering Holy Communion to the sick and dying, and also for the secondary, but still highly prized, purpose of adoring Christ present in the Eucharist.
The Roman Catholic Church considers the doctrine of transubstantiation, which is about what is changed, not about how the change occurs, the best defense against what it sees as the mutually opposed errors of, on the one hand, a merely figurative understanding of the Real Presence of Christ in the Eucharist (the change of the substance is real), and, on the other hand, an interpretation that would amount to cannibalistic eating of the flesh and corporal drinking of the blood of Christ (the accidents that remain are real, not an illusion).
(Edited by Moderator to conform with Forum Rules)
en.wikipedia.org/wiki/Transubstantiation