Expositor's Bible Commentary Commentary John 6:63

"It is the spirit that giveth life; the flesh profiteth nothing: the words that I have spoken unto you are spirit, are are life." — John 6:63 (ASV)

Jesus insists that the Holy Spirit is the one who imparts life to the believer; it is not transmitted by the process of physical eating. Jesus was saddened by the dullness of some of his disciples that prevented their truly believing in him.