To be sure, atheists aren't the first one to make the charge. 500 years ago Anabaptists denounced Catholics as "God gobblers", for example. Consider that the 16th century equivalent of an internet meme.
Not entirely fair, but, if you'll pardon the pun, does the charge have bite?
— Tentative Apologist (@RandalRauser) April 29, 2019
So here are my thoughts.
Let’s start with a definition of “cannibalism” courtesy of dictionary.com: “the eating of human flesh by another human being.”
What does the Catholic Church teach?
Next, we can cite some relevant passages from the Catechism of the Catholic Church:
1357 We carry out this command of the Lord by celebrating the memorial of his sacrifice. In so doing, we offer to the Father what he has himself given us: the gifts of his creation, bread and wine which, by the power of the Holy Spirit and by the words of Christ, have become the body and blood of Christ. Christ is thus really and mysteriously made present.
1367 The sacrifice of Christ and the sacrifice of the Eucharist are one single sacrifice: “The victim is one and the same: the same now offers through the ministry of priests, who then offered himself on the cross; only the manner of offering is different.” “In this divine sacrifice which is celebrated in the Mass, the same Christ who offered himself once in a bloody manner on the altar of the cross is contained and is offered in an unbloody manner.”
1374 The mode of Christ’s presence under the Eucharistic species is unique. It raises the Eucharist above all the sacraments as “the perfection of the spiritual life and the end to which all the sacraments tend.”199 In the most blessed sacrament of the Eucharist “the body and blood, together with the soul and divinity, of our Lord Jesus Christ and, therefore, the whole Christ is truly, really, and substantially contained.”200 “This presence is called ‘real’ – by which is not intended to exclude the other types of presence as if they could not be ‘real’ too, but because it is presence in the fullest sense: that is to say, it is a substantial presence by which Christ, God and man, makes himself wholly and entirely present.”201
1375 It is by the conversion of the bread and wine into Christ’s body and blood that Christ becomes present in this sacrament.
We see two things in these passages. First, the presence of Christ in the Eucharist is “unique” (1374). He is not present in this bodily form elsewhere in creation than in the sacrament. Second, he nonetheless really is present substantially as the bread and wine are converted into his very body and blood.
Karl Rahner and Herbert Vorgrimler state the doctrine of transubstantiation as follows:
Neo-Latin meaning “essential change”: at the consecration in the Mass the changing of the substance of bread and wine, by the power of God, into the substance of Christ’s Body and Blood, which thereby become present while the empirical realities as phenomena (Species) of bread and wine remain.” (“Transubstantiation,” Theological Dictionary, 466)
So that’s the doctrine of the Church.
Technical vs. Substantial Vegetarians and Non-Cannibals
If the wafer is transformed into the flesh of Christ, it would seem to follow necessarily that to consume the consecrated wafer is to consume the human flesh of Jesus Christ. Since a human person’s consumption of human flesh meets the definition of cannibalism, it follows trivially that the human person who consumes the human flesh of the consecrated wafer is, thereby, a cannibal.
It seems to me that the best way forward for the Catholic is to bite the bullet on this one. Yes, it is cannibalism. However, we must make an important distinction. While it is true that this act technically meets the definition of cannibalism, it is not cannibalistic in the substantial sense, and that’s the sense that matters.
So what’s the difference I am drawing between technical and substantial? That difference is rooted in the standard social function of the term “cannibal”. Consider, by analogy, the term “vegetarian”. To be a vegetarian is to abstain from the consumption of all animal matter (i.e. meat) in one’s diet.
While that is the standard definition of vegetarian, I would argue that it can also be viewed as the technical application of the term. By contrast, the substantial application of the term vegetarian is somewhat more narrow and pertains to abstaining from the consumption of all animal matter in one’s diet that once constituted part of an animal. There are at least two reasons for this dietary restriction: consuming that animal matter is complicit in the infliction of unjust suffering upon animals and it also exacts a disproportionate environmental cost. These concerns are the real motivation for censuring the consumption of animal matter.
And so, what if a person could consume meat wholly apart from any animal suffering or disproportionate environmental cost? I am thinking specifically of animal matter which has been cultivated in a laboratory such that this meat never formed part of the body of a sentient, living organism. Instead, it was cultivated from cells in a petri dish. (I’m assuming the cellular base was originally collected in a wholly ethical way consistent with vegetarian concern to avoid animal suffering.) While the consumption of this meat would technically violate the vegetarian identity, I would submit that it would be consistent with the substantial motivations behind (most) vegetarianism: i.e. the avoidance of animal suffering and disproportionate environmental cost of meat production.
From that perspective, the person who eats only lab meat may technically be violating the definition of vegetarianism, but they nonetheless meet the substantial definition and its underlying moral concerns. And thus, while this person may not be a technical vegetarian, they retain substantially a vegetarian.
The same point can be made with respect to cannibalism and the Eucharist. Non-cannibalism eschews the cannibalistic act because that act involves inflicting suffering upon human persons and devaluing human personhood and the body by way of consumption of that body. But those strictures assume that the matter which is consumed once formed part of a living human person’s body.
This is not true of the Eucharist. Thus, while these elements may technically become one with the body and blood of Christ, they were never part of the body of the living human person Jesus. In that sense, the consecration of Eucharistic elements in the Mass is analogous to the growing of new meat in a laboratory. And the consumption of the Eucharist avoids the social censure of cannibalism in the same way that the consumption of lab-grown meat avoids the social censure of carnivory.
To conclude, just as the person who restricts themselves to lab-grown meat may meet the substantial definition of being a vegetarian, so the person who restricts themselves to Eucharistic elements may meet the substantial definition of being a non-cannibal. And so, the cannibal charge may be good for a cheap shot in a meme, but as a significant objection to Catholicism, it lacks a substantial bite.