What's wrong with consubstantiation—the view that the Eucharist is both the body and blood of Christ and bread and wine? Isn't that more consistent with the Incarnation, and isn't transubstantiation almost Gnostic, even Docetist?
Superficially, consubstantiation might seem more "incarnational" than transubstantiation, but there’s a catch. For the Eucharist to be both Jesus Christ and bread and wine, as Jesus is both God and man, Jesus would have to unite the nature of bread to himself as he united human nature to himself. It would amount to a new incarnation, a new hypostatic union. We would confess a Lord who is truly God, truly man, and truly pastry. This would demean and trivialize the significance of our Lord’s assuming our human nature.
Furthermore, such a reprise of the Incarnation would not accomplish what the Eucharist is all about: It would not make present the human body and blood of Christ. If the Second Person of the Trinity were to acquire a new, confectionery nature, this new nature would have no direct relationship to Jesus’ human nature. He would be present in the Eucharist in his divinity and his breadness, but not his humanity. His human body, born of Mary, crucified on the cross, raised from the dead, and ascended into glory, would be uninvolved.
This is not, of course, what consubstantiationists believe. They picture Christ in his divinity and his humanity juxtaposed with bread and wine, not becoming them. But this is not the incarnational principle. It is more like Nestorianism. It makes the Eucharist an amalgam of Jesus and bread, just as Nestorius made Jesus an amalgam of God and man without truly uniting the two natures in one person.
The authentic Catholic understanding of the Eucharist, by contrast, is not a repetition of the Incarnation but an extension of it. Christ is not hypostatically united to bread, but the one hypostatic union of divinity and humanity is presented to us under the appearances of bread and wine. It is not a new, independent redemptive act, but the making present of the one redemption accomplished by Christ in his Incarnation, Crucifixion, Resurrection, and Ascension.