John 6 is about faith/belief in Jesus. Not eating/drinking the flesh and blood.
Catholics understand that the Jews in the Scriptures and the current protestors do not have Faith in the Words and teachings of Jesus about our need to eat and drink His Body and Blood for eternal life.
While the Jews understood the meaning of words of Jesus, the protestors made up their own meaning and personal interpretation.
I praise the word of God;
I trust in God, I do not fear.
What can mere flesh do to me? Psalms 56