"The words that I speak to you are spirit, and are life." Peter knew the issue was not a cannibalistic ritual he needed for eternal Life, it was just what Jesus had, the Words of Spirit and Life. May God have mercy upon those int he catholic religion who never comprehend that and instead continue to participate in the pagan rite of eating the god of Catholicism.
I wonder, then, why he did not go and explain that to those disciples that left.
"It is the spirit who gives life. The flesh profits nothing. The words that I speak to you are spirit, and are life."
The words that he spoke were that we must eat his flesh and drink his blood. The contrast here is between what he has just spoken and the crowds desire for earthly bread. Previously, in John 3, when Jesus speaks of the contrast between flesh and spirit he also says that we must be born of water and Spirit.
Jesus answered, “Amen, amen, I say to you, no one can enter the kingdom of God without being born of water and Spirit. What is born of flesh is flesh and what is born of spirit is spirit." (John 3:5-6)Jesus uses sacramental signs to impart spiritual gifts. The contrast between flesh and spirit is that between earthly and heavenly. The crowds were following him because they wanted earthly bread to fill their stomachs. It is not a denial of the reality of the Eucharist. Otherwise, you would have Jesus go on at great length how we must eat his flesh and drink his blood, even to the point of have many of his disciples leave, and then just dismiss this with a single verse.