Hi
Rawb,
That is a wonderful question, and it’s one that I am very interested in. Although
Vox Borealis is sure that someone has a more lucid explanation, I’m not sure I am that person who will rise to the level of lucidity

. But maybe between you, Vox, and I (and other contributers) we can begin to shed some light on this most interesting topic.
I was just wondering why we believe that the bread is both the body and blood and the wine is both body and blood? Jesus said “This is my body” in reference to the bread and “this is my blood” in reference to the wine.
The fact that each element or species of the Eucharist is both the Body and the Blood of Christ is a natural and inevitable conclusion drawn from the doctrine of the Real Presence of Christ in the eucharist.
As Vox pointed out above, each species is the Body, Blood, Soul and Divinity of Christ, in other words, the whole and entire Christ.
But the Body and Blood of Christ (as well as His Soul and Divnity) are made present in different ways in each of the consecrated elements.
By the divine power, the sacramental presence of Christ becomes present when the bread is converted into the Body of Christ, and the wine is converted into the Blood of Christ. But it is also an article of faith that we receive the living Christ, now risen and glorified. Since Christ is alive, His Body and Blood are joined,
by reason of concomitance (i.e. a “natural connection”)
Furthermore, by reason of concomitance, Christ’s Soul is joined to His Body and Blood (since He is alive). And still further, by reason of the Hypostatic Union, Christ’s Divinity is united to His Body, Blood, and Soul.
So, to recap: Christ’s Body becomes present through the Sacramental action of the consecration of the bread, converted into His Body. His Blood, Soul and Divinity* are present through concomitance.
Likewise, with the conversion of the wine to His Blood. His Body, Soul and Divinity* are present through concomitance.
We can see the beauty and fittingness in this: the sacramental signs are fulfilled, bread becomes Body, and wine becomes Blood. In addition we can see how this doctrine preserves the
reality of the Real Presence: since Christ is really present as He is now, and since Christ is alive (and can never die again), wherever His Body is so must be His Blood, Soul and Divinity. Wherever His Blood is, so must be His Body, Soul and Divinity.
I think I might remember it has something to do with combating a heresy?
You are probably thinking about the heresy of
Utraquism, which posited the neccesity of receiving communion under both kinds in order to be saved. This heresy, as well as the erroneous belief of the Nestorians, does seem to deny the article of faith that Christ is whole and entire under each form.
So,
Rawb and
Vox, what do you think? I’m always happy when threads like this pop up because I usually learn quite a bit, and I hope that the more we can grow in the knowledge of the Real Presence of Christ in the Eucharist the more we will learn to love Him.
God Bless,
VC
*Strictly speaking, Christ’s Divinity is present by virtue of the Hypostatic Union, and not a
natural concomitance.