One of the most misquoted passages among Catholics is John 6. Where they stop and start varies depending on the Catholic you're dealing with, and the particular argument they are making from the passage, but the gist of it is that they think when Jesus says we must eat Him to be saved, He was speaking quite literally.
There are multiple problems with this claim. The first and most obvious is that Jesus Himself denies this. In verse 63, He tells His disciples He was speaking spiritually, and in verse 47, He tells us the spiritual truth He was trying to convey: "Amen, Amen, I say to you, whoever believes has eternal life". We also know that this event happened at a time when Jesus did not speak plainly (John 16:25-33), especially to the people (Matthew 13:10-17; 33-35), with the specific intention that the scriptures be fulfilled which declare that the Messiah would speak in parables that confuse those who see and hear Him.
There are several other problems I could address, and have done so several times in the past (see the Reaching Catholics section), but the greatest failure of the Catholic argument is that Jesus says whoever eats His flesh and drinks His blood will never perish. He even compared it to those who ate manna in the desert and died. This obviously can't be about physical death, because that comes to us all. Enoch and Elijah are the only people in history who did not die. And so it must refer to spiritual death. But how many times do Catholics have to take the Eucharist? Many. Even then, they are not guaranteed salvation.
Such is the failure of the Catholic "Gospel". It has no power to save. There is no hope in it. No security. Whereas the Christian Gospel gives assurance, the Catholic Gospel gives even less hope than the Law that was fulfilled in Christ. The high priests had to offer sacrifice after sacrifice after sacrifice, just as Catholics take Eucharist after Eucharist after Eucharist, but Jesus sacrificed Himself once for all (Hebrews 7:27). Thus, Christians have real hope. All we have to do is believe. Amen, amen, I say to you, whoever believes has eternal life.