WHY CAN'T NON-CATHOLICS RECEIVE THE EUCHARIST?
Catholicism regards the Eucharist as a one flesh bond between Jesus and the Church, like that of a husband and a wife. Catholics believe that consummating a marriage bond is appropriate after a covenant has been solemnly made in public before witnesses. So also with the Eucharist; it would be inappropriate to receive the Eucharist without having passed through the proper preparation that requires faith in the Real Presence and knowledge of what is required for anyone to worthily receive it.
JESUS IS A VINE (JN 15: 1) & A DOOR (JN 10: 9) BUT NOT LITERALLY; WHY THEN SHOULD WE BELIEVE THAT JESUS IS SPEAKING LITERALLY WHEN HE SAYS: "THIS IS MY BODY"?
When Jesus calls Himself the vine or the door, the context suggests that He is speaking figuratively: Jn 16: 29 implies that vine is used in a figurative sense; Jn 10: 6 expressly calls the illustration of the door a 'figurative speech'. In contrast, everything about the Last Supper suggests that Jesus is speaking literally (Luke 22: 19-20); see also His statement in John 6: 57 that we must eat Him. No-one appears to have left Jesus because of His symbolic vine teaching, but they did with His literal teaching of the Eucharist in John 6: 66. Also, grammatically, "I am the vine" cannot be literally true because a man cannot literally be a plant. Jesus did not say, "Bread is my body" (which would be a contradiction); He said "This is my body." The "this" is unspecified until it is identified as Christ's body. So, there is no contradiction in taking this statement literally, as there would be if we took literally the statements about the "vine" & the "door".
* Please note that this text should be read in the context of the whole work and in recognition of the appropriate paragraphs of the Catechism of the Catholic Church highlighted in the index.