How does Roman Catholicism justify a literal meaning of 'Hoc est meum corpus'?
hoc est meum corpus [Vulgate - Jerome 382/Clementine 1592]
this is my body
Biblically it's comes from the lips of Jesus Christ...
Your specific question:
1.How then, at that instant, could bread have been - literally - his own real body. ?
CCC1333 At the heart of the Eucharistic celebration are the bread and wine that, by the words of Christ and the invocation of the Holy Spirit, become Christ's Body and Blood. Faithful to the Lord's command the Church continues to do, in his memory and until his glorious return, what he did on the eve of his Passion: "He took bread. . . ." "He took the cup filled with wine. . . ." The signs of bread and wine become, in a way surpassing understanding, the Body and Blood of Christ; they continue also to signify the goodness of creation. Thus in the Offertory we give thanks to the Creator for bread and wine,154 fruit of the "work of human hands," but above all as "fruit of the earth" and "of the vine" - gifts of the Creator. The Church sees in the gesture of the king-priest Melchizedek, who "brought out bread and wine," a prefiguring of her own offering.155
Biblically it's comes from the lips of Jesus Christ...
T"....his bread is my flesh, which I will give for the life of the world.”
From the very word of Jesus Christ:
" And he took bread, and when he had given thanks he broke it and gave it to them, saying, "This is my body which is given for you. Do this in remembrance of me." And likewise the cup after supper, saying, "This cup which is poured out for you is the New Covenant in my blood."(Matthew26:26-27)
Now the BIG QUESTION that your trying to point is if the Transubstantiation occurs during the Last Suffer...
Venerable Arch. Sheen said the Last Supper and the Calvary are the same. One is the unbloody sacrifice instituted for all of us and the other is the bloody sacrifice in Calvary At the Foot of the Cross.
The Priest and the offered sacrificed are one and the same Jesus Christ.
When Jesus said to the Apostles" what I have done, Do this in Remembrace of Me".Venerable Sheen said Jesus empowered the Apostles to celebrate the Holy Sacrifice with a vision of the actual sacrifice at Calvary.
CCC1333 teaches the an ordained priest invoking the Holy Spirit turns the bread & wine in real flesh and blood...Then if this is our faith, How can we doubts if Jesus is the one who said it in the Last Supper Himself.
Now, do we believe that the bread & wine at the Last Suffer become the true flesh and blood of Jesus?
If we have faith in the words of Jesus that it is His flesh & blood, then it is really true by virtue of our living faith in Him.
As God said "so My word that proceeds from My mouth will not return to Me empty, but it will accomplish what I please, and it will prosper where I send it.
Other Supporting views:
To give light on the issue...let's hear from the Jewish Bible Scholar talking on the Passover meal from the point of view of the Old Testament.
The significant observation was Jesus did not drink the final cup of wine which is customary for the Jews in celebrating the Passover meal...so based on the Jewish customs the Passover Meal Jesus by not drinking the final cup of wine it is not yet finished, it is incomplete...
This is affirmed in the Garden of Gethsemane where Jesus said "... “My Father, if it is possible, let this cup pass from Me. Yet not as I will, but as You will.” (Jesus still not drinking the wine)
The Holy Sacrifice was finished at Calvary AT THE FOOT OF THE CROSS, after Jesus had drank the wine.
John 19:28-30 King James Version (KJV)
28 After this, Jesus knowing that all things were now accomplished, that the scripture might be fulfilled, saith, I thirst.
29 Now there was set a vessel full of vinegar: and they filled a spunge with vinegar, and put it upon hyssop, and put it to his mouth.
30 When Jesus therefore had received the vinegar, he said, "IT IS FINISHED" and he bowed his head, and gave up the ghost.
The whole and complete sacrifice was now offered.
Dr. Brant Pitre shines fresh light on the Last Supper by looking at it through Jewish eyes.
What was Jesus' purpose in instituting the Eucharist during the feast of Passover? And most important of all, what did Jesus mean when he said, "This is my body... This is my blood"?
"We are to believe that the Eucharist began in the womb of the Virgin Mary; that the flesh which the Son of God received from His Mother at the Incarnation is the same flesh into which He changed bread at the Last Supper; that the blood He received from His Mother is the same blood into which He changed wine at the Last Supper. Had she not given Him His flesh and blood there could not be a Eucharist."
In closing, I will give a mystery of the Holy Eucharist when the First Mass was celebrated in the Upper Room...
The Holy Eucharist is the body,blood,soul & divinity of our Glorified Resurrected Jesus Christ sacramentally instituted for us to transform our flesh & blood into intimate union/oneness with Jesus Christ eternally.
Now the Mystery in the Upper Room is... what happen when an Immaculate Body (Flesh & Blood) of the Theotokos received the Holy Eucharist?
Did She received a Transformation similar to the Resurrected Glorified Body of Jesus Christ?...Did the Theotokos transformed to Glorified Body that is worthy to enter the Kingdom of God even while still alive here on earth?...
This is the Mystery on Infinite Mystical Union of the the Holy Spirit to Her Spouse/Advocate who became the Spirit of Truth...(but I wont expound it here)
This is the POWER OF THE HOLY EUCHARIST and it's transforming power experienced by the Theotokos in Her Immaculate Body!(A Mystery worth discovering...)