Nowadays we Catholics tend to read this passage from St. John, indeed this whole chapter, as if it were first and foremost about the Eucharist --- even though that would make no sense in the context of the Gospel or even in the unfolding of Christ's life.  This Gospel text is purely and simply about willingness to believe in Jesus, to eventually believe that the human Jesus of Nazareth is also the God of Israel, and only in later centuries did we find in this text any resonance with the Eucharist and about that aspect of our relationship to Christ. 

One of the principal themes of John's Gospel is this question of us reaching a decision or judgement about Christ and who He is in Himself and in relation to us.  John's Jesus is often speaking about making that judgement in the formal legal terms of witnesses, testimony, and even sending us this “Paraclete” --- and one of the meanings of this Greek word was pretty much equivalent to “defense attorney” or “champion before the law.”

In this passage Christ is almost exclusively asking His listeners to judge whether or not He has come from the Father in a very special and unique role, although He does not define that role beyond it being a nourishment for believers that far outstrips what even the miraculous manna could do for the Jews in the desert. 

This passage is in chapter 6 of John's Gospel, and at this point Christ is only asking people to advance in their trust in Him; we still have almost 5 more chapters to go before we reach the end of chapter 11, the beginning of Christ's last days, which include the actual foundation of the Eucharist, and then another 9 chapters before we reach the end of the original gospel, where Thomas is the first to proclaim, in so many words, that Jesus is God.