Why is partaking of Holy Eucharist not the same as cannibalism?
Ironically, as early as Pliny the Younger (c. AD 110), there are indications that rumours were circulating about the early Christians which included allusions to the crime of cannibalism (Ep. 10.96.7; cf. Tacitus, Ann. 15.44.2). Around AD 150, Justin Martyr mentions the accusations of eating of human flesh (Apol. 1.26.7). Theophilus of Antioch (ca. AD 180-185) wrote of "the prevalent rumor wherewith godless lips falsely accuse us, who are worshipers of God and are called Christians, . . . that we eat human flesh." [To Autolycus, Bk III, Ch. IV].
So, it seems I'm in good company. ;)
So why isn't partaking the Holy Eucharist the same thing as cannibalism?
Cannibalism = (1) eating of human flesh by a human being; (2) eating of the flesh of an animal by another animal of the same kind.
Holy Eucharist (partaking of) = eating the accidents of bread and wine, having the substance of the body, blood, soul, and divinity of a divine person, Jesus Christ.
Cannibalism is evil because it causes unjust injury to human beings (i.e. unjustly injuring them so as to use them for food). Historically, cannibalism has been done ritually after victory in battle, when they ritually eat the flesh of their enemy, for example.
Christians do not do injury to a living Incarnate God by sacramentally consuming the accidents of bread and wine whose substance metaphysically becomes the incarnate body and blood of the same living God.
Cannibalism is an injustice to humanity, whereas Holy Eucharist is faithful obedience to Jesus Christ who told his disciples to eat his body "as true food."
Longer, metaphysical explanation...
If one thinks transubstantiation amounts to cannibalism, this may be due to not having understood what substance is, and how it is related to accidents, metaphysically speaking.
I'll begin by defining some theological and philosophical terms...
Transubstantiation - The complete change of the substance of bread and wine into the substance of Christ's body and blood by a validly ordained priest during the consecration at Mass, so that only the accidents of bread and wine remain.... After transubstantiation, the accidents of bread and wine do not inhere in any subject or substance whatever. Yet they are not make-believe; they are sustained in existence by divine power. (Etym. Latin trans-, so as to change + substantia, substance: transubstantiatio, change of substance.) [Fr. John Hardon, Modern Catholic Dictionary].
Metaphysics = the philosophical study of the nature of being (ontology), the nature of the universe (cosmology), and the nature of knowledge (epistemology).
Accidents = (1) a nonessential property or quality of an entity. [Merriam-Webster] (2) things whose essence naturally requires that they exist in another being. Accidents are also called the appearances, species, or properties of a thing. These may be either physical, such as quantity, or modal, such as size or shape. [Fr. John Hardon, Modern Catholic Dictionary].
Substance = (1) essential nature; ultimate reality that underlies all outward manifestations [Merriam Webster]. (2) A being whose essence requires that it exist in itself.... It is commonly distinguished from an accident, whose essence is to exist in another, that is, in a substance. (Etym. Latin substantia, that which stands under, principle, foundation.) [Fr. John Hardon, Modern Catholic Dictionary].
Something naturally must either exist in its own right, such as water, a tree, a cat: or else it naturally must exist in something else, such as color or shape. The substance is the essence, the nature, of a thing which exists in its own right. The accidents depend on it for their existence and their operation.
We can't see a substance or touch it or taste it. Yet, it is real nevertheless.
Comparatively, Scripture describes angels who "appear" to be men. Does that necessarily mean that they stopped being, in "substance" angels? No. God can change the accident of a thing without changing its substance. Likewise, God can change the substance of a thing without changing its accident. Angels may appear (i.e. accident) to be men, but remain in "substance" angels. Likewise, bread may appear (accident) to remain bread, yet in substance become the body, blood, soul, and Divinity of Jesus Christ. Both would have to be miracle. We believe in such miracles.
Moreover, when we naturally eat something, we eat its "accidents" which inheres to its "substance." In the case of mere bread, both the accidents and the substance are bread. However, like the appearance of holy angels as men, Holy Eucharist is something miraculous, something supernatural, the real (substantive) presence of God in a way beyond physics (meta-physics). Once the bread and wine are consecrated, it is no longer normal bread and wine, but as Jesus called it, "my body." Consequently, when we eat the consecrated bread, that which Jesus calls in John ch. 6 "true food," that food which gives eternal life, we really eat the accidents of bread and the miraculous or supernatural substance of Christ's body (ie. his real body, blood, soul and divinity).
The dogma of transubstantiation teaches that the whole substance of bread and the whole substance of wine is changed into Christ's body, blood, soul and divinity, leaving the accidents of bread and wine unaffected. Reason, of course, can't prove that this happens. But it is not evidently against reason either; it is above reason, it is beyond physics (like all miracles). Our senses, being confined to phenomena, cannot detect the change: we know it only by faith in God's word.
"Lord, in my zeal for the love of truth, let me not forget the truth about love."
-- St. Thomas Aquinas