A Catholic source:"This is seen in their interpretation of the key biblical passage, chapter six of John’s Gospel, in which Christ speaks about the sacrament that will be instituted at the Last Supper." An Episcopalian Source: Receiving our Lord's Body and Blood in the Eucharist is necessary for eternal salvation..  "So Jesus said to them, Truly, Truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you. . . .  he who eats this bread will live forever." (v:53-58)
From A Protestant source: "Here, then, is some of the deepest New Testament teaching about the Eucharist. The focus of this teaching is on sacrifice and shared life. These are inseparable since there is no sharing of life without the laying down of life. The once-for-all sacrifice of Christ is the pouring out of his life for the life of the world, bringing forgiveness and a new power of life. That sacrifice also shows us the deepest reality about God--his love--and about life: all true life is sacrificial. Life is a matter of exchange: my life for yours, yours for mine. In this sacrificial web of exchange we find the communion, the community, of the Godhead. At Eucharist we receive into ourselves, into our bodies and souls, the life-giving power of God, and precisely by eating and drinking we proclaim the Lord's once-for-all death until he comes."

---

So Jesus said to them, Truly, truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you;(v:53) This is the maximum offense to Jews, for it is written:Whoever eats any blood, that person shall be cut off from his people(Lev.7:27) ,That is why he said it - to offend them. Why? The context is very clear. He had fed 5000 men with five loaves and two fish the day before and they wanted to forceably make him their king. He withdrew into the hills by himself and, next day, they found him and he rebuked them: Truly, truly, I say to you, you seek me, not because you saw signs, but because you ate your fill of the loaves (v:26).They had bad motives and were trying to force him to their will. Therefore he tested all of them, including the Twelve. So he said to them, Do you take offense at this (v:61)? After this, many of his disciples drew back and no longer went about with him (v.66). That was the intended result. The bad motives left. Then, to the Twelve, Will you also go away (v.66)? Peter said,"Lord, to whom shall we go? You have the words of eternal life; and we have believed....that you are the Holy One of God." There is not one word here pertaining to the Eucharist! He was screening his disciples - nothing more. By implication, we also learn that Jesus never instituted the Eucharist, so called, for he would have know that his words here would be confused with it and he would have chosen some other screen.