The claim that one must literally eat the body and blood of Christ, that even a particle of the consecrated wafer is held to wholly contain Christ, (CCC #1377) and is "able to sanctify thousands of thousands and is sufficient to afford life to those who eat of it,” (St. Ephrem, Hymni et sermons, IV, 4) is a result of attempting to apply Jn. 6:53 to the Lord's Supper under a literal hermenuetic.
In both cases Caths boast of going by the plain literal meaning of the text, but which would mean that eating what the Lord said is "my body which is broken for you" (1Cor. 11:24) "my flesh, which I will give for the life of the world," (Jn. 6:51) actualy was that bloody body, not something that somehow had its essence changed so that it looked, tasted, behaved like bread/wine but really was flesh and blood under the appearence of bread and wine, with bread alone also being flesh and blood. And so that the Lord could digest like bread and wine while yet sitting before them.
Likewise, if Caths are to be consistent with Jn. 6:53 being literal, and with the absolute unequivocal imperitive nature of other "verily, verily" statement, then they must hold that none of those who deny the Cath "real presence" are born again, and can have eternal life.
But they cannot, unless they are one of the sects that constitutes Catholicism.
I have tried to get them to read Luke’s account, where The Lord said plainly, “This cup is the new covenant in my blood.” With the same mind of John 6 the cup is the covenant.