He asked them that after he explained the metaphor in John 6:63, "
The Spirit alone gives life. The flesh is of no avail.
The WORDS that I have spoken to you are both Spirit and life." Then they confirmed their belief in his explanation after he asked if they would leave him too. And Simon Peter replied, " Lord to whom shall we go?
Your WORDS are WORDS of eternal life. We have faith and know that you are the Holy One of God." John 6:68
"The Holy One of God" being the prophet, a Jewish man, described in Deuteronomy 18:18
_____________________________________________________________________________
"He censures those who understood our Lord's words after the letter, (literally), as if they were to eat the carnal body.
He expounds the spiritual thing which gives life as to be understood by the text: "the WORDS that I speak unto you, are both spirit and life."
His WORD is the life-giving principle, therefore he called his flesh by the same appellation, so we are to "devour Him with the ear, ruminate on Him with the understanding, and digest Him by faith."
Tertullian.
Ante-Nicene Fathers, Vol. III