This is your source so find in the bible the covenant of David and find how it relates to the Cup of the covenant of david.
The "covenant of David" refers to the OT "Davidic Covenant". Plug either phrase into google and you'll get all sorts of links....with appropriate chapter and verse.
Jesus' followers, no doubt, hoped/believed that he was the fulfillment of the promised messiah. That part, I can understand to some degree. However, the whole NT eucharist practice boggles my mind in light of Jewish thinking and practices with regards to blood. Since Jews would have been revolted by the idea of "drinking blood", I can see why there would be no mention of such a thing in the earlier writings (the Didache). Drinking blood was practiced in ancient cult worship, but NOT among the Jews.
"Finally, the Didache tell how the initiate, who fasts and prays before being baptized, would have learned how sharing in this simple meal of bread and wine links the human family gathered for worship with "God, our Father," and with "Jesus, [his] servant" (or his "child," as the Greek term pais may be translated). And by "breaking bread" together, his people celebrate the way God has brought together people who once were scattered, and has joined them as one:
"As this broken bread was scattered upon the mountains but was brought together and became one loaf, so let your people be gathered together from the ends of the earth into thy kingdom."
Those speaking this prayer in unison ended by calling - in an ancient Aramaic phrase some Christians invoke to this day - for the imminent coming of the Lord: "Let grace come, and let this world pass away....Maran atha! [Our Lord, come!] Amen." According to Draper's analysis, these are Jews who rever Jesus as "God's servant" and believe that his coming signals Israel's restoration at the end of time.
But other early followers of Jesus, like the majority ever since, saw the sacred meal in a much stranger - even macabre - way: as eating human flesh and drinking human blood. Only twenty years after Jesus' death, Paul declared that Jesus himself commanded his followers to do this. Paul, like the gospels of Mark, Matthew, and Luke, tells how on the night Jesus was betreayed,
"while [the disciples] were eating, [Jesus] took bread, and after blessing it he broke it, gave it to them, and said, "Take: this is my body." Then he took a cup, and after giving thanks he gave it to them, and all of the drank from it, and he said to them, "This is my blood."
Tetullian satirized the reaction of outsiders to this practice: "We are accused of observing a sacred ritual in which we kill a little child and eat it." He writes,
"No doubt [the Christian] would say, "You must get a child still very young, who does not know what it means to die, and can smile under your knife; and bread to collect the gushing blood....Come, plunge your knife into the infant....Or, if that is someone else's job, simply stand before a human being dying before it has really lived....Take the fresh young blood, saturate your bread with it, and eat freely."
Despite his sarcasm, Tertullian cannot dispel the shocking fact that the Christian "mystery" invites initiatiates to eat human flesh - even if only symbolically. Pagans might be repelled by the practice of instructing newcomers to drink wine as human blood, but devout Jews, whose very definition of kosher (pure) food requires that it be drained of all blood, would be especially disgusted. "
Source: Beyond Belief - The Secret Gospel of Thomas, by Elaine Pagels (pages 17 - 19)