The doctrine of the Real Presence asserts that in the Holy Eucharist Jesus is literally and wholly present—body and blood, soul and divinity—under the appearances of bread and wine. Many Protestants attack this doctrine as “unbiblical,” but the Bible is forthright in declaring it (cf. 1 Cor. 10:16–17, 11:23–29; and, most forcefully, John 6:32–71).
The early Church Fathers interpreted these passages literally. In summarizing the early Fathers’ teachings on Christ’s Real Presence, renowned Protestant historian of the early Church J. N. D. Kelly, writes: “Eucharistic teaching, it should be understood at the outset, was in general unquestioningly realist, i.e., the consecrated bread and wine were taken to be, and were treated and designated as, the Savior’s body and blood” (Early Christian Doctrines, 440).
From the Church’s early days, the Fathers referred to Christ’s presence in the Eucharist. Kelly writes: “Ignatius roundly declares that . . . [t]he bread is the flesh of Jesus, the cup his blood. Clearly he intends this realism to be taken strictly, for he makes it the basis of his argument against the Docetists’ denial of the reality of Christ’s body. . . . Irenaeus teaches that the bread and wine are really the Lord’s body and blood. His witness is, indeed, all the more impressive because he produces it quite incidentally while refuting the Gnostic and Docetic rejection of the Lord’s real humanity” (ibid., 197–98).
https://www.usccb.org/resources/The%20R ... harist.pdf
In the celebration of the Eucharist, the glorified Christ becomes present under the appearances of bread and wine in a way that is unique,
a way that is uniquely suited to the Eucharist. In the Church's traditional theological language, in the act of consecration during the
Eucharist the "substance" of the bread and wine is changed by the power of the Holy Spirit into the "substance" of the Body and Blood of
Jesus Christ. At the same time, the "accidents" or appearances of bread and wine remain. "Substance" and "accident" are here used as
philosophical terms that have been adapted by great medieval theologians such as St. Thomas Aquinas in their efforts to understand and
explain the faith. Such terms are used to convey the fact that what appears to be bread and wine in every way (at the level of "accidents"
or physical attributes - that is, what can be seen, touched, tasted, or measured) in fact is now the Body and Blood of Christ (at the level of
"substance" or deepest reality). This change at the level of substance from bread and wine into the Body and Blood of Christ is called
"transubstantiation." According to Catholic faith, we can speak of the Real Presence of Christ in the Eucharist because this
transubstantiation has occurred (cf. Catechism, no. 1376). This is a great mystery of our faith—we can only know it from Christ's teaching
given us in the Scriptures and in the Tradition of the Church. Every other change that occurs in the world involves a change in accidents
or characteristics. Sometimes the accidents change while the substance remains the same. For example, when a child reaches
adulthood, the characteristics of the human person change in many ways, but the adult remains the same person—the same substance.
At other times, the substance and the accidents both change. For example, when a person eats an apple, the apple is incorporated into
the body of that person—is changed into the body of that person. When this change of substance occurs, however, the accidents or
characteristics of the apple do not remain. As the apple is changed into the body of the person, it takes on the accidents or
characteristics of the body of that person. Christ's presence in the Eucharist is unique in that, even though the consecrated bread and
wine truly are in substance the Body and Blood of Christ, they have none of the accidents or characteristics of a human body, but only
those of bread and wine.
