Well, let me begin with moving this post/question to it's own blog entry so as to not be confused with the original post where you asked the question. Feel free to add more to the combox on this entry.
I know this post isn't on transubstantiation, but I saw your response to Rhoblogy's post on transubstantiation. While I'm not Catholic and don't hold to the doctrine, I'm also not really fundamentally against it. I don't see it in scripture, but I don't know that there's anything terrible about holding to it, since I think the Eucharist, whatever the true nature of it is, is a very intimate and spiritual moment with Christ.
That said, I am interested in the formulation. William Lane Craig, in one of his Defenders podcasts, talked about the issue and mentioned something interesting a Catholic told him about Transubstantiation and how it made sense in the Catholic's mind. Craig asked him how Christ's body had not been exhausted throughout the years. I can't remember the answer. Could you extrapolate on it for me? The answer, as I remember, had something to do with the nature of the substance consumed.
Secondly, I can respect that you do not hold to the doctrine - but don't outright reject it. I grew up a Lutheran, so it wasn't a real huge leap for me to move to the Catholic understanding. I had always understood a "Real Presence" of sorts in the Lutheran Communion, now I have a fuller understanding that it IS His body and IS His blood - not that His body and blood are "with" the bread and wine. The fundamental principle and scriptural support for this lies in the fact that Jesus Himself declared not that the bread would have a Real Presence WITH it, but that the bread now IS His body. St. Augustine said it well:
"I promised you [new Christians], who have now been baptized, a sermon in which I would explain the sacrament of the Lord's Table. . . . That bread which you see on the altar, having been sanctified by the word of God, is the body of Christ. That chalice, or rather, what is in that chalice, having been sanctified by the word of God, is the blood of Christ" (Sermons 227 [A.D. 411]).And to bolster those statements from St. Augustine, let us also look to what St. Ignatius of Antioch said (and keep in mind, St. Ignatius was a disciple to St. John the Apostle, so Ignatius received this information first-hand from the Apostle who wrote John 6!).
"I have no taste for corruptible food nor for the pleasures of this life. I desire the bread of God, which is the flesh of Jesus Christ, who was of the seed of David; and for drink I desire his blood, which is love incorruptible" (Letter to the Romans 7:3 [A.D. 110]).
So, you can see this is not some new innovation of Catholics after the time of the Protestant Reformation. Catholics have maintained this belief in the Real Presence since the very words of Christ declared this to be so.
Now, how is the body of Christ not all "used up" by now? I would point you back to John 6, as I did Alan (Rhology) in my original comment. In the first part of John 6 we have the pre-figuring of just how Jesus can be in many places at one time! Jesus took a few fishes and loaves of bread and miraculously fed over 5000 people! How could so few fishes and loaves be passed around to over 5000 people? It is a mystery of faith! We believe not because we can fully understand and explain the miracles of God, no! We believe because we have FAITH. Jesus demonstrated that He is capable of such a miracle - so when He took bread and declared it to BE His body - then it IS His body! How? We can't say - but we accept Jesus at His Word here.
Why do we believe Jesus is speaking literally here and not figuratively? Because, going back to John 6, Jesus COMMANDS us - MULTIPLE TIMES that we MUST eat His body and drink His blood or we have NO LIFE in us! This caused the Jews to grumble - and even many of his disciples, those who had been following Him and learning from Him, to "turn and walk with Him no more." If this were just a figurative parable here, why would Jesus allow His DISCIPLES to walk away with a false understanding of what He meant? But He did let them walk away, why? Because they DID understand Him correctly but did not have the FAITH necessary to BELIEVE HIM.
I hope this helps you to understand this teaching better, and pray that God gives you the faith necessary to believe Him - without question - on this matter. Just as Jesus turned to The Twelve and asked, "Will you also leave?" St. Peter answered Him, "To whom shall we go? You have the words of everlasting life!"
PS- This is also answered by Matthew Bellisario here.