And do we not all eat that same spiritual meat? and do we not all drink that same spiritual drink? and are we not all made to drink into the same one Spirit? (1Cor 10:3-4, 1Cor 12:13). Therefore it concerns eating the spiritual food of the Word, which is also likened unto Living Water, which is also the blood of our atonement; cleansing the man from the inside, for that holy Testimony of Messiah in the Gospel accounts was purchased with the price of his own innocent blood: signed, sealed by the Father, and delivered, having been offered up as a willing sacrifice to whosoever will come and drink of the fountain of the water of life freely.
I'm not sure that one can equate "living water" with "the blood of our atonement." Seems to me that the water and the blood are separate substances. John 19:34 and 1 John 5:6 distinguishes between them.
A careful reading of John 6 shows that Jesus was fully prepared to have all of his followers abandon him, if it came to that, over his claim that he was giving them his flesh to eat. Never once did he say to his followers, "Hey, don't go, I was only speaking in a symbolic or spiritual sense. I didn't literally mean what I actually said!"
Paul also reiterates the Real Presence of Christ in the Lord's Supper whenever he spoke of it.
This was also confessed by all Christian leaders after the time of the Apostles, such as Ignatius of Antioch, Justin Martyr, Irenaeus, John Chrysostom, Augustine, etc. For example:
Ignatius of Antioch: "Consider how contrary to the mind of God are the heterodox in regard to the grace of God which has come to us. They have no regard for charity, none for the widow, the orphan, the oppressed, none for the man in prison, the hungry or the thirsty. They abstain from the Eucharist and from prayer, because they do not admit that the Eucharist is the flesh of our Savior Jesus Christ, the flesh which suffered for our sins and which the Father, in His graciousness, raised from the dead."
Justin Martyr: "This food we call the Eucharist, of which no one is allowed to partake except one who believes that the things we teach are true, and has received the washing for forgiveness of sins and for rebirth, and who lives as Christ handed down to us. For we do not receive these things as common bread or common drink; but as Jesus Christ our Savior being incarnate by God's Word took flesh and blood for our salvation, so also we have been taught that the food consecrated by the Word of prayer which comes from him, from which our flesh and blood are nourished by transformation, is the flesh and blood of that incarnate Jesus."
Irenaeus: "So then, if the mixed cup and the manufactured bread receive the Word of God and become the Eucharist, that is to say, the Blood and Body of Christ, which fortify and build up the substance of our flesh, how can these people claim that the flesh is incapable of receiving God's gift of eternal life, when it is nourished by Christ's Blood and Body and is His member? As the blessed apostle says in his letter to the Ephesians, 'For we are members of His Body, of His flesh and of His bones' (Eph. 5:30). He is not talking about some kind of 'spiritual' and 'invisible' man, 'for a spirit does not have flesh an bones' (Lk. 24:39). No, he is talking of the organism possessed by a real human being, composed of flesh and nerves and bones. It is this which is nourished by the cup which is His Blood, and is fortified by the bread which is His Body. The stem of the vine takes root in the earth and eventually bears fruit, and 'the grain of wheat falls into the earth' (Jn. 12:24), dissolves, rises again, multiplied by the all-containing Spirit of God, and finally after skilled processing, is put to human use. These two then receive the Word of God and become the Eucharist, which is the Body and Blood of Christ."
John Chrysostom: "It is not the power of man which makes what is put before us the Body and Blood of Christ, but the power of Christ Himself who was crucified for us. The priest standing there in the place of Christ says these words but their power and grace are from God. 'This is My Body,' he says, and these words transform what lies before him."
Augustine: "You ought to know what you have received, what you are going to receive, and what you ought to receive daily. That Bread which you see on the altar, having been sanctified by the word of God, is the Body of Christ. The chalice, or rather, what is in that chalice, having been sanctified by the word of God, is the Blood of Christ."
Many more quotes here: http://www.therealpresence.org/eucharst/father/a5.html