Transubstantiation is the Catholic doctrine that, during the Eucharist, the substance of bread and wine changes into the substance of Christ's Body and Blood, while the appearances of bread and wine remain. Catholics believe this happens at the consecration, so Jesus becomes truly present under the form of the host and wine.
What happens during the consecration?
In Catholic teaching, "substance" describes what a thing is, and "accidents" are its observable qualities. During the consecration, the entire substance of the bread and wine becomes the Body and Blood of Christ. The appearances remain unchanged: the consecrated host looks, tastes, and smells like bread.
This is not a mixture of Jesus and bread, and no visible change takes place. Catholics teach that Christ is really and personally present in the consecrated bread and wine. They also believe Christ remains present in hosts reserved in the tabernacle and brought out for Eucharistic adoration. The tabernacle is often a locked box near the altar, and a sanctuary lamp burns near it when the reserved sacrament is present.
Where does the teaching come from?
The Latin word means "change of substance." Catholic theologians developed this precise language during the Middle Ages, and the Council of Trent affirmed and defined the doctrine in 1551. It remains a settled teaching of the Roman Catholic Church and of the Eastern Catholic Churches, whose liturgies often describe the Eucharist in the language of mystery or change.
The Catechism of the Catholic Church explains this doctrine in paragraphs 1374 through 1379, including the reasons the Church honors Christ present in the consecrated host.
How do other churches explain the Lord's Supper?
Eastern Orthodox churches describe a mysterious transformation of the bread and wine but generally avoid the Western category of transubstantiation. Lutheran churches commonly teach consubstantialiation, meaning Christ is present in, with, and under the bread and wine. Reformed churches generally describe the elements as signs or seals of Christ's promise, rather than objects literally changed. Baptists and many other churches observe the Lord's Supper as a memorial, remembering Christ's death without describing a literal change in the elements.
What should a visitor expect?
In a Catholic Mass, the priest prays over the bread and wine at the moment of consecration, and the church teaches that transubstantiation occurs then. In the United States, Catholics may receive Communion kneeling or standing, on the tongue or in the hand. Some parishes also set aside a place for quiet adoration of the reserved sacrament before or after Mass.