Jesus told Satan that Man can’t live on bread alone but on every Word that comes from the mouth of God. The Every Word is Jesus Himself. After He had fed the 5000 on bread and fish (not bread alone) He told them that in order to have Life in them they must eat His Body and drink His Blood. A whole load of people walked away, and Jesus didn’t call them back. It is thought that this teaching was the one that turned Judas.
At the Last Supper Jesus finally showed the apostles how His Bride, the Church, was to receive His Body and Blood as He spoke His Word over bread and wine (the offering of the priest Melchizedek) and said “This is My Body” and “This is My Blood”.
Judas received the first sacrilegious Communion. St Paul has to warn against receiving the Holy Eucharist unworthily making the receiver “guilty of the blood” – meaning guilty of the equivalent of murder. (Blood guilt in the OT is murder). From his warning we see that there were sacrilegious Communions even in the early years of the Church.
Misunderstanding of the Eucharist was pretty standard among the pagans. They accused Christians of being cannibals who eat their god. It wasn’t until much later that the Church was accused of trying to re-crucify Christ at each Mass. Nevertheless there has been lack of Faith and understanding of the Eucharist and sacrilegious reception since the beginning. So God has given us signs of His power in the Eucharist through great Saints and Blesseds who have lived for long periods of time on the Eucharist alone.
St Catherine of Siena lived on the Eucharist alone for the last few years of her life. Many of her companions, and her mother tried to get her to eat but it made her so sick they gave up. She also had the stigmata, but she had requested that it not be visible and God granted her this. It didn’t show until just after her death, when the wounds then appeared.
Other saints include St Catherine of Genoa who lived through the fasting times of Lent and Advent on only the Eucharist. St Joseph Cupertino lived for 5 years without food apart from the Eucharist.
But the most well known, most scientifically documented and most rigorously recorded case has to be that of Blessed Alexandrina da Costa who spent 13 years without food or drink but for the Eucharist. She is considered the 4th Fatima Seer. She died in 1955 but her canonisation process is already well underway.
Christ promised that He would not leave us orphans and He has always kept that promise. He provides proofs and signs of His Presence to help all who doubt, and strengthen the faith of those who receive Him. In these saints and Blessed we see that those who eat His body and Drink His Blood really do have life in them.