I usually look to John 6, since it starts off around a passover (hinting that this is connected to the Paschal Mystery), and that Jesus says over and over that we must eat His Flesh and drink His Blood, and that He didn’t correct His disciples (which He always did), and a majority of them walked away.
There’s also the passage in 1 Corinthians 10 when Saint Paul tells us that this is a communion in the Body and Blood, and in the next chapter warns us of profaning the Body and Blood of the Lord and that we can eat and drink Judgement to ourselves if we consume unworthily, and that some have died as a result of this.
But one website I went to quoted Hebrews 6 on this matter, that when we sin we crucify Jesus anew and put Him to shame, so they said it could be a figurative speech?
But Saint Paul does compare the Eucharist to the sacrifices of the Jews and Pagans in 1 Corinthians 10, bringing back the theology that in order for the crucifixion to have been a sacrifice, the Sedar must have been a sacrifice, too, as Jesus interrupted it and said after drinking sour wine in John’s Gospel: It is finished (and a bunch of theologians believe that He was referring to the Paschal Sacrifice, not Redemption, since Saint Paul said that Jesus was raised for our Justification). Then we add in the prophesy of Malachi, who spoke of how there would be a pure sacrifice offered all the time all over the world.
But there’s another question: how do we know that the sacrifice predicted in Malachi is singular in the sense that it is Calvary re-presented, or sacrifices of personal contrite hearts?