Roman Catholics tend to think it is highly significant that Jesus said that the cup (meaning its contents – they never seem to misunderstand that use of non-literal language) is “my blood.” Recall that Jesus said:
Matthew 26:28 For this is my blood of the new testament, which is shed for many for the remission of sins.
Mark 14:24 And he said unto them, This is my blood of the new testament, which is shed for many.
Luke 22:20 Likewise also the cup after supper, saying, This cup is the new testament in my blood, which is shed for you.
1 Corinthians 11:25 After the same manner also he took the cup, when he had supped, saying, this cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me.
Instead of understanding this according to its most obvious metaphorical meaning, Roman Catholics try to insist that we should interpret it in some kind of quasi-literal sense. But Jesus, the Son of David, is using metaphor in much the same way that his father David used it:
2 Samuel 23:13-17And three of the thirty chief went down, and came to David in the harvest time unto the cave of Adullam: and the troop of the Philistines pitched in the valley of Rephaim. And David was then in an hold, and the garrison of the Philistines was then in Bethlehem.
And David longed, and said, “Oh that one would give me drink of the water of the well of Bethlehem, which is by the gate!”
And the three mighty men brake through the host of the Philistines, and drew water out of the well of Bethlehem, that was by the gate, and took it, and brought it to David: nevertheless he would not drink thereof, but poured it out unto the Lord.
And he said, “Be it far from me, O Lord, that I should do this: is not this the blood of the men that went in jeopardy of their lives?” therefore he would not drink it. These things did these three mighty men.
For David, the water represented the potential death of his men. For the Son of David, who turned water into wine, the wine represented his own death, which we should remember, as often as we drink it. Unless you think David was saying that the water was transubstantiated into blood — but who would be so dull-witted as to think that?