Why do Catholics believe they recieve the living Christ in Communion?
We believe in the Real Presence of Jesus Christ in the Holy Eucharist because He promised to give us His Flesh as food and His Blood as drink. In order to see this we must turn to the sixth chapter of St. John's Gospel.
That evening Jesus worked yet another preparatory miracle. Jesus told His apostles to get in their boat at Tiberius and sail to Capernaum without Him. In the night (vs. 16-22) the sea became rough because of a high wind. Suddenly, the apostles saw Jesus walking on the water toward the boat. Then, once Jesus was in the boat, it miraculously landed at the shore of Capernaum.
The next morning (vs. 22-24), the people who had eaten the loaves at Tiberius could not find Jesus. They had seen the apostles sail away without Him, and they couldn't understand why they couldn't find Him. So the people went around to Capernaum to find Him. When the people found Jesus at Capernaum, they asked Him how He got there. Jesus cut to the heart of the matter when he said,
Jesus went on to tell them to work for the spiritual food that would get them to heaven, instead of the temporal food that spoils. He told them to do this by believing in Him (v. 29).
"Truly, truly, I say to you, you seek me, not because you saw signs, but because you ate your fill of the loaves" (v. 26).
These disciples who ate the miraculous bread the day before wanted a sign from Him to prove He was worthy of their belief. They told Jesus that Moses had given their fathers manna from heaven to eat. They wanted to know if He could top that, despite the miracles they'd already seen!
He went on to explain through verse forty that He was the bread sent from heaven by the Father.
"I am the bread of life; he who comes to me shall not hunger, and he who believes in me shall never thirst" (v. 35).
Up to that point Jesus' followers understood Him to be speaking symbolically. Jesus took this misconception away from them right away. He went on to tell them that He was the bread they would have to eat to inherit eternal life.
"The Jews then disputed among themselves, saying, 'How can this man give us His flesh to eat?' So Jesus said to them, 'Truly, truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you; he who eats my flesh and drinks my bloods eternal life, and I will raise him up on the last day. For my flesh is food indeed, and my blood is drink indeed. He who eats my flesh and drinks my blood abides in me, and I in him. As the living Father sent me, and I live because of the Father, so he who eats me will live because of me" (vs. 52-57).
No longer did Jesus' followers believe He was being symbolic. They now understood Him to be speaking literally. He said, "my flesh id food indeed, and my blood is drink indeed" (v. 55); the word "indeed" made His statement imperative. He prefaced His entire statement with the phrase "truly, truly", which was always used to emphasize the importance of what He was about to teach.
The followers' literal understanding of what Jesus said repulsed them,
"After this many of his disciples drew back and no longer went about with him" (v. 66).
As the people walked away, rather than trying to keep them there by telling them they had misinterpreted Him, Jesus turned toward His apostles and asked,
But the ever affirming Peter replied with a question of his own.
"Will you also go away?"
"Lord, to whom shall we go? You have the words of eternal life; and we have believed, and have come to know, that you are the Holy One of God" (vs. 67-69).
The apostles had been with Him from the beginning. They also understood what He said to be literal. They did not know how He would do what He said, but they believed that He would eventually show them.
Jesus finally explained the how at the Last Supper. In Luke 22:15 Jesus said,
He had earnestly desired to give them His Flesh and Blood since He first made the promise. In verses sixteen through twenty Jesus proceeded to give what He had promised.
"I have earnestly desired to eat this Passover with you before I suffer."
These passages, as well as many others, explain why Catholics believe in the Real Presence of Jesus Christ in the Holy Eucharist. The short of it is, we believe in the Real Presence because Jesus said so!