Suppose I rushed up to you, all excited, and said, "The most incredible thing has happened! Sit down, because you are going to be astounded! I know some of this is going to be hard to believe, but you've got to understand, I'm not making this up! I saw it with my own eyes. And what I have seen with my own eyes, what my hands have handled, is the very word of life itself! God has become man, died on the Cross and been raised from the dead! Now He makes Himself available to us in the most intimate communion possible in this life, through the sacrament of His Most Holy Body and Blood! I saw it, I tell you. This is no myth! This is really happened!"
What would you think if I then sat down and proceeded, in answer to your many questions about this hard-to-buy story, to invent "sayings and acts" of Jesus out of complete whole cloth, and attribute them to Christ?
I suspect your reaction would be much like most people's. You'd recognize me as, at best deluded, and (more honestly) as a pathological liar.
Yet when John tells us that Jesus said, "The bread that I will give is My flesh for the life of the world," (Jn 6:41-51), Andrew Greeley helpfully informs us that this is, in his opinion, exactly what the Evangelist is doing: "One must not take this passage as a description of an actual dialogue between Jesus and some of those who followed Him. Rather it doubtless refers to a difficulty in St. John's community over the Eucharist and the presence of Jesus in the Eucharist, a difficulty which has plagued the Church through its history, mostly because have tried to reduce mystery to prose, to explain the inexplicable."
Happily, the Church (as distinct from Fr. Greeley) does not ask us to believe that the Gospels are simply fictional channelings of "What Jesus would have said to the Johannine community, according to the Johannine Editorial Staff that Pasted Together the Gospel of John." The Church, instead, asks us to believe that "Since everything asserted by the inspired authors or sacred writers must be held to be asserted by the Holy Spirit, it follows that the books of Scripture must be acknowledged as teaching solidly, faithfully and without error that truth which God wanted put into the sacred writings for the sake of our salvation" (Dei Verbum).
Some folks do not see what the problem is. They see no substantial difference between saying the Gospel writers paraphrased Jesus and saying they made up entire stories and sayings, affirmed them as gospel truth and told us to believe them on pain of hellfire. Why, they ask, couldn't this process be guided by the Holy Spirit too?
The basic answer is that this process is normally known as "lying." But the Gospels are not a lie undertaken for our good. The Gospels are telling us what Jesus said and did.
It is perfectly true that the Gospels do not always give us the ipsissima verbi or "exact words" of Jesus. The Gospels are, after all, Greek accounts of conversations that largely took place in Aramaic. Moreover, the materials in the Gospels are arranged by the writer with real editorial control in order to make particular points. So when Matthew takes certain sayings of Jesus and informs us that they were said on a mountain, he has a theological purpose: he wants us to understand that Jesus is the New Moses for the Jewish people. Likewise, when Luke's Jesus speaks the same batch of sayings on a level place, he wants us to understand that Jesus is the Son of Man and the Savior of all people who has joined us in our low estate. That's because the purpose of the Gospels is to arrange real historical materials to make a theological point, not to chronicle Jesus' every word and deed. Indeed, the Gospels barely cover 100 days in the entire life of Jesus. But what they cover actually happened.
That said, it is simply false to posit the choice "Either every single word of the Gospels exactly reproduces the tape-recorded sayings of Christ or else we are free to believe that the Apostles just made up a bunch of junk Jesus never said and attributed it to Him." The teaching of the Church is basically that John 6 preserves the substance of real things said by Jesus in a conversation with the crowd at Capernaum. The teaching of the Church is not "John just made up a bunch of baloney and attributed it to Jesus to get himself out of a pastoral jam." It is that the Gospels are reliable accounts of the life, teaching, death, and resurrection of Jesus Christ.