"For to believe in Him is to eat the living bread. "
I certainly read this as "Believing in him is to eat the living bread" or "By eating the living bread you are believing in him". There is nothing to prevent me from reading it that way, other than IF what you say about "believe and you have eaten already" is true.
"9. And now addressing the few that remained: Then said Jesus to the twelve (namely, those twelve who remained), Will ye also, said He, go away? Not even Judas departed. But it was already manifest to the Lord why he remained: to us he was made manifest afterwards. Peter answered in behalf of all, one for many, unity for the collective whole: Then Simon Peter answered Him, Lord, to whom shall we go? You drive us from You; give us Your other self. To whom shall we go? If we abandon You, to whom shall we go? You have the words of eternal life. See how Peter, by the gift of God and the renewal of the Holy Spirit, understood Him. How other than because he believed? You have the words of eternal life. For You have eternal life in the ministration of Your body and blood. And we have believed and have known. Not have known and believed, but believed and known. For we believed in order to know; for if we wanted to know first, and then to believe, we should not be able either to know or to believe. What have we believed and known? That You are Christ, the Son of God; that is, that You are that very eternal life, and that You give in Your flesh and blood only that which You are.
Tractate 27, para 9, here. http://www.newadvent.org/fathers/1701027.htm
Like I said, everyone should read ALL his tractates and ask themselves "Does he sound more Catholic or Protestant here?"
I see no reason to accept your interpretation of "eat and you have believed already". That's the linchpin of your entire assertion about Augustine (and his teachings on the Eucharist). I see no reason to accept it because not only does reading more of Augustine than the quote "believe and you have eaten already" show more than a passing importance to the Eucharist, but even more importantly I don't recognize your authority to claim "believe and you have eaten already" defines all of what Augustine means about eating the sacrament.
The former must be read spiritually, and the latter is impossible, because Augustine says "Why ready teeth and stomach? Believe and you have eaten already." Thus faith precedes any possible physical eating, and therefore can't refer to the eucharist at all. You say this yourself in your other post, writing "he is not speaking about the Eucharist," not realizing that such an admission confirms my position. But note how you tried to claim that Augustine is not talking about eating Christ, as if he was making some general comment about food, and not giving instructions on how to eat Christ, thus you wrap yourself up in all sorts of trouble, because, quite clearly, Augustine is talking about eating and drinking Christ's body and blood.
But this is incompatible with the view that Christ is eaten through the Eucharist, confirmed partly even out of your own mouth. If Augustine is talking about Christ's body and blood, you can't claim that eating the eucharist is necessary to eat Christ. These things must be entirely exclusive, otherwise you contradict Augustine's words: "Believe and you have eaten already."
Thus my explanation of the difference between Augustine's teachings on the eucharist and his teachings on eating Christ through faith remain unmolested and supported by the other quotations I already made. I see no reason to accept it because not only does reading more of Augustine than the quote "believe and you have eaten already" show more than a passing importance to the Eucharist,
Well, if we read MORE of Augustine, my position only gets stronger. I've only shown you a small fraction of the quotations I have. Do you want the rest?