John 6.56 is one of today's Communion antiphons: Whoever eats my flesh and drinks my blood remains in me and I in him, says the Lord. If the consecrated bread and wine were merely symbolic representations of Jesus' flesh and blood, he would have simply said "Whoever eats this bread and drinks this cup . . . ." It's just common sense.
Here are the three verses preceding 56. Jesus said to them, "Amen, amen, I say to you, unless you eat the flesh of the Son of Man and drink his blood, you do not hav life within you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him on the last day. For my flesh is true food, and my blood is true drink. How can you eat the ordinary bread and drink the ordinary wine and have eternal life? Of course, you must believe that the bread and drink once consecrated do become the flesh and blood of the Lord. Anyway, the Apostles, the early Church, and the Catholic Church to this day have all believed in the Real Presence of Christ in the Holy Eucharist.