There are few times in Scripture where Jesus specifically says in direct order “Do This” or “Verily, Verily - Very Truely”. One such order, is in John 6: 53-59. He was so sincere about this order, that He allowed the majority of the crowd to disperse from Him and then questioned the Apostles, asking them if they wished to leave also.
This calling out of the Eucharist as cannibalism has existed since the Early Church - Tertullian (160 - 220) - Tertullian, The Apology, Chapter 7:1.
Read more: catholicfidelity.com/apologetics-topics/eucharist/questions-about-the-eucharist/ .
St. Justin Martyr wrote (148-155) also about the Real Presence.
“This food we call the Eucharist, of which no one is allowed to partake except one who believes that the things we teach are true, and has received the washing for forgiveness of sins and for rebirth, and who lives as Christ handed down to us. For we do not receive these things as common bread or common drink; but as Jesus Christ our Savior being incarnate by God’s Word took flesh and blood for our salvation, so also we have been taught that the food consecrated by the Word of prayer which comes from him, from which our flesh and blood are nourished by transformation, is the flesh and blood of that incarnate Jesus.”
“First Apology”, Ch. 66, inter A.D. 148-155.
Perhaps a scholar on Luther, could explain why and when Luther lost the faith of the Church concerning the Real Presence.
Luther’s loss of faith in the Eucharist (which is based back to both Sacred Scripture and Sacred Tradtion, is the real question that needs answered.