The charge of cannibalism is exactly the error the early Christians knew the Roman pagans would make
For the early Christians, the charge was almost certainly due to a misunderstanding of the language of the metaphor, just as it happened in John 6, because in those early days they had no record of introducing complex theories of substance that would even remotely suggest actual cannibalism.
But it remains a valid question whether transubstantiation as dogma constitutes a formal approval of cannibalism. I say this because substance cannot logically be entirely separated from the accident that expresses it. This is not a matter of faith or lack of it. This is a matter of words meaning things. God can do anything, except for that which would involve Him in self-contradiction.
Consider the following problem:
We are constantly reminded here that "Real Presence" means the body and blood are "really present" in the host, that "is" must be taken literally, not metaphorically, etc. But even if we grant that (which we do not), what have we granted? Real in what way? Literal how? We must drill deeper to specifics, or we will never know what those words mean.
For example, if we say real (or literal) means physical, we are told, at least by some, that such realness/literalness does not actually mean "physical" as we normally mean physical, but it means "substance" (ousia).
Well and good, but what is "substance?" If we are given a quick and glib answer, we can be sure the person has no concept of the history of that word. For those interested in a short introduction, the following article is helpful:
http://plato.stanford.edu/entries/substance/#MedAccSub
1) Background
A popular analogy might help illustrate the true complexity of this topic. If I say "river," most folks know what that means as a concept, a large flow of water from one point to another along a constrained path. But if I walk up to a specific stretch of the Mississippi, and put my foot in the rushing water, the water molecules that wash over my foot are constantly changing. Yet the river keeps supplying new water molecules, and so persists in having an identity as a river.
This persistence of identity is substance, at least in the Greek system. The water moving past my foot at any given time is called an "accident." Accidents are the things that constantly change, yet if that change is regulated so as to remain an expression of "river," then the river's identity
as a river is retained.
But now let's say we want the river to retain it's "riverness," but we want to express this particular river without those properties that make it a river. Immediately we see a problem. If we remove, say, the property of flowing water, then do we still have a river? No we do not. The properties that are necessary to complete the definition of "river" are essential to it being a real river. The property cannot be removed without losing the substance as well. The river will lose its identity without those accidents.
2) The Proposed Aquinan Solution
Notice the problem this presents to transubstantiation. If all accidents must have a subject, i.e., a substance of which they are the expression, then removal of the substance of bread and wine would take the accidents away as well. There would be no bread and wine to express. Only body and blood. The perception of bread and wine depends on there really being bread and wine present.
But Aquinas to the rescue. His supreme innovation? "Real accidents." Remember that accidents, as a matter of definition, must have what is called a subject on which they inherently depend for their existence. This subject is the substance. But if, as transubstantiation requires, we must be able to separate a substance from the accidents that express it, then the accident must be of a kind that supposedly can be separated from the substance to which it is giving physical reality.
So Aquinas posits that there can be such accidents as can be separated from their subjects, but only by miracle. These are called "real accidents," because, in theory, they can exist in the world independently of a particular subject. They are substance swappable. But only by miracle.
Now, to make this easier, Aquinas makes dimensionality the key accident, the "one ring that rules them all," because no other accident imposes itself on space-time if it is just an infinitely small, mathematical point. So if you short-circuit dimensionality with just this one miracle, you can, so the story goes, have the entire substance of something physical, yet in no physical sense as we understand physicality, because all physicality we know has dimensionality.
3) Does it work?
Not so well:
A) For example, serious problems arise when one tries to use the words of this equation consistently. If we say that a substance is a substance because it is has independent being, yet we also say that some class of accidents exist where they can have being apart from their originating substance, what we are really saying is the accident is a substance in it's own right. Now if that doesn't leave you feeling confused, I don't know what will. It tangles everything up. If we cannot even differentiate between accident and substance, because the criteria are up for grabs, the whole exercise is lost to the void.
B) Furthermore, even if we simply defer the unanswered questions about to which category "real accidents" belong, we still have a fundamental problem in logic. No miracle ever purported to change the rules of logic. The resurrection is possible, not because God tosses out rationality to accomplish it, but because His power to create life entails infinite rationality. He knows how to do this. No true rule of logic is ever broken to do a true miracle. Else God would be fighting Himself, as He is the source of all rationality.
And that's the rub. "Real accidents" are an exercise in trying to say what is logically impossible. Consider this example. We have a 50 miles long river. But we say that, by a miracle, the "substance" of the river has been disappeared. Gone. In its place is put the "substance" of a four inch lead cube. The "accidents" all taken together give the appearance of a 50 miles long river. No matter how many times we measure it, it always comes out 50 miles long.
To what do these accidents refer as their substance? Nothing. Perhaps you are thinking its the cube, but no, they have no referent substance. This is because in the substance-accident pair of the four inch lead cube, the accidents that express its "four inch lead cubeness" are what refer back to that substance. But the river, no longer identifying a river, still identifies as a river, but isn't one, because its accidents, the rushing water, the channel in the ground, etc., all refer back to ... nothing. So an identification is occurring, but it is false. There is no river. Who are you going to believe? Me or your lying eyes? (Marx Brothers).
Remember, this is not my idea. This is Aquinas:
Therefore it follows that the accidents continue in this sacrament without a subject. This can be done by Divine power: for since an effect depends more upon the first cause than on the second, God Who is the first cause both of substance and accident, can by His unlimited power preserve an accident in existence when the substance is withdrawn whereby it was preserved in existence as by its proper cause, just as without natural causes He can produce other effects of natural causes, even as He formed a human body in the Virgin's womb, "without the seed of man" (Hymn for Christmas, First Vespers).
Available here: http://www.newadvent.org/summa/4077.htm
Aquinas goes to great lengths to justify this expression of nothingness. The trick is to call it what we "know" the substance to be. In our case of the river, we just call it a lead cube, because that's what's "really" there. And if you doubt such a thing can be, why, you have denied the power exemplified in the incarnation. Yet his comparison of these two events is as wrongheaded as it can be. Read the context at the link. He doesn't even attempt to show, at least not here, nor anywhere I know about, how a true physical miracle like the incarnation, which produced real, testable, physical effects (as well as spiritual), is even remotely like a "miracle" that consists of ignoring definitions to achieve it's alleged effect.
C) Nevertheless, this alleged miracle of distorted logic is, in theory, the escape hatch that prevents cannibalism. One cannot digest an infinitely small point, I suppose. So the excuse goes that because Christ's physicality in the host is "real" (though we have no idea what that means now) but cannot be metabolized as ordinary food, it does not constitute cannibalism.
But this is redefining cannibalism for convenience. The law prohibiting the intake of blood is sufficiently general to include any ordinary form of eating human blood, regardless of whether digestion is successful, or the victim is alive or dead, or any other number of qualifiers. One simply cannot do an act of drinking human blood without violating the law.
If the defense is raised that no physical human substance is physically consumed at all, then we have a complete collapse of the whole transubstantiation argument. If the body and blood in substance are untouchable, undigestable, such that their physicality never, ever connects with our physicality, then yes, we have this way avoided the real physical intake of any real physical humanity, and so avoided cannibalism. But now we are no longer understanding "this IS my body" in any way that that the terms "literal" or "physical" are ever used. If there is no physical contact between two physical humans, we might as well be talking about two phantasms that pass through each other with no contact whatsoever. Surely this is not the desired endpoint for transubstantiationists.
So the problem comes down to this dilemma. If you use as your defense against cannibalism the substance/accidence barrier, to show a lack of true physical contact, then you no longer have transubstatiation as advertised, because you are no longer truly eating the body and blood of Christ. If you insist that somehow the physical transaction does occur, so that in some way you are really eating the body and the blood in any physical sense, then you have admitted cannibalism.
4) An ancient "apologetics tract" that reflects an entirely different defense against the charge of cannibalism:
I find it interesting that when early Christians addressed this charge of cannibalism, they did not go down the road of making complex arguments about substance. Instead, they would just flat out deny they were eating body or blood of anyone, which is not consistent with the substance theory of transubstantiation. Consider this excerpt from an early "apologetic tract," written as a hypothetical dialog between a Christian and a Pagan:
And now I should wish to meet him who says or believes that we are initiated by the slaughter and blood of an infant. Think you that it can be possible for so tender, so little a body to receive those fatal wounds; for any one to shed, pour forth, and drain that new blood of a youngling, and of a man scarcely come into existence? No one can believe this, except one who can dare to do it. And I see that you at one time expose your begotten children to wild beasts and to birds; at another, that you crush them when strangled with a miserable kind of death. There are some women who, by drinking medical preparations, extinguish the source of the future man in their very bowels, and thus commit a parricide before they bring forth. And these things assuredly come down from the teaching of your gods. For Saturn did not expose his children, but devoured them. With reason were infants sacrificed to him by parents in some parts of Africa, caresses and kisses repressing their crying, that a weeping victim might not be sacrificed. Moreover, among the Tauri of Pontus, and to the Egyptian Busiris, it was a sacred rite to immolate their guests, and for the Galli to slaughter to Mercury human, or rather inhuman, sacrifices. The Roman sacrificers buried living a Greek man and a Greek woman, a Gallic man and a Gallic woman; and to this day, Jupiter Latiaris is worshipped by them with murder; and, what is worthy of the son of Saturn, he is gorged with the blood of an evil and criminal man. I believe that he himself taught Catiline to conspire under a compact of blood, and Bellona to steep her sacred rites with a draught of human gore, and taught men to heal epilepsy with the blood of a man, that is, with a worse disease. They also are not unlike to him who devour the wild beasts from the arena, besmeared and stained with blood, or fattened with the limbs or the entrails of men. To us it is not lawful either to see or to hear of homicide; and so much do we shrink from human blood, that we do not use the blood even of eatable animals in our food.
Available here: http://www.newadvent.org/fathers/0410.htm
This document is believed to be written near the end of the Second Century. The writer's rebuttal to the charge of cannibalism is two-fold. First, he says his pagan friend suspects the Christians of child-murder and blood drinking because the pagans are accustomed to such violence and so can imagine it in others. In modern terms, he is saying his pagan friend is guilty of projection.
The second line of defense is that as Christians they have a solid aversion to consuming blood of any kind, animals of course, but especially human.
So all in all, he is not making a nuanced argument that they partake of a specialized kind of blood-drinking, that is both truly consuming Christ's blood in some literal way and yet not consuming Christ's blood in any literal way that would constitute cannibalism. He never even reaches the question of the sacred meal. He just makes a simple denial. Christians don't consume human blood. Period.
Surely this is a defense that any good non-denominational Christian might raise even today. But then by the end of the Second Century, the Roman denomination with all its distinctives had not yet emerged, and we were all just Christians then.
BTW, I apologize for the length of the post. I tried shortening it several times, but this is what it grew to. I hope you found it useful, despite the length.
Peace,
SR