Protestants who deny the Real Presence are sometimes branded as Gnostics. They "spiritualize" the sacraments. Reduce them to "mere" symbols or "nude signs". They're allergic to any connection between grace and physicality. So goes the allegation.
Before getting to the main point, I'd like to make a few preliminary observations:
1. Professing Christians who affirm the Real Presence vary in the specificity of what they affirm. Some leave a lot of room for "mystery". In principle, if you affirm the Real Presence, then that commits you to one of two basic options:
i) The communion elements are actually the physical body of Christ (or Jesus in toto) rather than bread and wine. That's either/or.
ii) The communion elements are the physical body of Christ in addition to bread and wine. That's both/and.
2. Some Christians don't try to explain it. They punt to "mystery". They just call it a miracle.
i) In principle, I can respect that. However, that's an argument from authority, so it's only as good as the ostensible authority. Only as good as their prooftexts or ecclesiastical authority.
ii) Another problem with that appeal is that the NT nowhere depicts the Eucharist as a miracle. It doesn't use miracle terminology. It doesn't show Christians reacting to the Eucharist as a miracle–unlike how people Scripture react to public miracles. So there's no indication that the Eucharist is miraculous.
3. Let's examine the both/and option. On that view, the Eucharist is real bread and wine as well as the real body of Christ. However, it only appears to be bread and wine. The appearance is true insofar as the bread and wine are real enough, and that's in part what the Eucharist is.
But, conversely, by not appearing to be something it is, it appears to be something it's not–for the most important thing about the Eucharist (on this view) is the physical body of Christ (or Jesus in toto), yet it doesn't seem to be that at all. It doesn't seem to be anything other than bread and wine. So the appearance is deceptive.
So why isn't that Gnostic? We might call this epistemological Gnosticism. Even though there's more to the Eucharist than meets the eye (or other senses, or chemical analysis), all you can perceive is bread and wine.
Suppose, during WWII, a French art collector covered his Monets and Renoirs with cheap canvas paintings to disguise them from the Nazis. They look like cheap paintings. And what the viewer sees is real enough. But right beneath the canvas of the cheap painting is a priceless Monet or Renoir.
4. Suppose, for the sake of argument, we propose a model for the both/and option. The bread and wine exists in our universe, while Jesus exists in a parallel universe. At the moment when a priest pronounces the words of consecration, an invisible wormhole opens up connecting the two.
That still doesn't explain how different individuals can ingest one and the same body. So let's say the body of Jesus exists in multiple parallel worlds. The body of Jesus is fissioned in a multiverse. Each communicant ingests the body of Jesus, which has infinitely many counterparts in parallel worlds. Like Hilbert's Hotel, you never run out of bodies to ingest at communion.
Even if the body/and option is something like that, the parallel universe is indetectable from our side of the event horizon. So there's a chasm between appearance and reality. Once again, that's awfully Gnostic.
5. Now let's explore the either/or option. That's classically represented by the Roman Catholic dogma of transubstantiation.
i) Some Catholic apologists might try to deflect criticism by claiming it's a miracle and invoke mystery. I don't object to that in general. However, that won't work in this case, in part because Rome did not content itself with calling it a miracle and leaving it at that. Rather, transubstantiation is a philosophical explanation. A rational attempt to account for how it can actually be Jesus despite all appearances to the contrary. In that event, the explanation is properly subject to rational scrutiny.
ii) In fairness to Aquinas, if you're going to say the communion elements are really Jesus, then that commits you to something like transubstantiation. Even if you don't subscribe to Aristotelian physics or Thomistic metaphysics, your dogma requires you to drive a wedge between the primary properties and the secondary properties. So you will end up with something similar (if not identical) to transubstantiation. Given the either/or option, there's no affinity between the phenomenal qualia of the bread and wine and what the communicant is actually ingesting. Jesus takes the place of the communion elements. He supplants the communion elements.
It seems to be bread and wine all the way down. According to our five unaided senses, it's bread and wine. According to chemical analysis, it's bread and wine. Put the wafer under an electronic microscope, and it seems to be just that. So the empirical properties are systematically misleading. Delusive.
Moreover, this extends beyond epistemological Gnosticism to embrace metaphysical (or ontological) Gnosticism. Just as Jesus only appeared to be human (a la Gnosticism), and only appeared to die on the cross (a la Gnosticism), the Eucharist only appears to be bread and wine, while the reality is something entirely different.