Maybe it is His Spirit that quickens our spirit. Do not think His flesh quickens my flesh or spirit. Yes He incarnated and died as flesh and spirit but the spirit quickeneth the whole matter of understanding.
The whole point is that our human flesh doesn’t gain anything from the Holy Eucharist because it isn’t meant to. It’s not supposed to feed our flesh, but it does feed our soul. His Body and Blood, Soul and Divinity are mystically fused with our own soul. This is how we truly become one with Him, both physically and spiritually. This prepares our soul for attaining eternal life with Him, and gives us the spiritual strength to follow Him all the way to the end.
As far as what they did not believe, there is no evidence that they did not believe the eating . They did not believe He was the Messiah and His mission. They believed in a Messiah of their own misunderstanding. They did not know Jesus.“Depart from me I, never knew you”. They departed.
How can you say there is no evidence that they left because they didn’t believe what He said about eating His flesh? Why would they say, “[53] The Jews therefore strove among themselves, saying:
How can this man give us his flesh to eat?”
They were astounded by what He said! They didn’t understand Him, and thought He was crazy! So, He continued, “[54] Then Jesus said to them:
Amen, amen I say unto you: Except you eat the flesh of the Son of man, and drink his blood, you shall not have life in you. [55] He that eateth my flesh, and drinketh my blood, hath everlasting life: and I will raise him up in the last day.”
Then, they were convinced that He was crazy! That’s why they left, “[61] Many therefore of his disciples, hearing it, said: This saying is hard, and who can hear it?”
These were people that had followed Him for a while and had witnessed His miracles, and listened to Him preach. They weren’t only the people that were following Him for free food. The Gospel clearly says they were “disciples”. When the Gospels refer to the regular crowds that gathered to listen to Him, they just refer to them as “the people”. But, His disciples were those that were already Baptized because they liked what He said and did. But, they were like the seed that landed by the wayside that Jesus spoke of in the parable, “Matthew 13:[18] Hear you therefore the parable of the sower. [19] When any one heareth the word of the kingdom, and understandeth it not, there cometh the wicked one, and catcheth away that which was sown in his heart: this is he that received the seed by the way side.”
Their lack of understanding caused them to lose the faith that they once had. But Peter and the Apostles had a much stronger faith, so even though they didn’t really understand all that Jesus said, they still believed that it was the truth.
Jesus was certainly not saying that we should symbolically “eat His words” by hearing the Gospel, as some have suggested. We should certainly listen to the Gospels, but I wouldn’t refer to that as “eating His words”. To me, that phrase has a very negative connotation to it. That’s why I, personally, wouldn’t use it. And, this discourse was much more specific. He repeated His point many times, and emphasized the fact that He was talking about His followers actually eating His Flesh and drinking His Blood, but He also said that it would be in the form of “the bread that I will give”. This is exactly what happened at the Last Supper when He said, “
this is my body”, so it all makes perfect sense.