Growing up made easier
In order to more fully understand Jesus, one must understand His role as the Lamb of God.
We start in the book of Genesis, when Abraham is tested by God:
Some time afterward, God put Abraham to the test and said to him: Abraham!
"Here I am!" he replied.
Then God said: Take your son Isaac, your only one, whom you love, and go to the land of Moriah. There offer him up as a burnt offering on one of the heights that I will point out to you.
Early the next morning Abraham saddled his donkey, took with him two of his servants and his son Isaac, and after cutting the wood for the burnt offering, set out for the place of which God had told him. On the third day Abraham caught sight of the place from a distance. Abraham said to his servants: "Stay here with the donkey, while the boy and I go on over there. We will worship and then come back to you."
So Abraham took the wood for the burnt offering and laid it on his son Isaac, while he himself carried the fire and the knife. As the two walked on together, Isaac spoke to his father Abraham. "Father!" he said. "Here I am," he replied. Isaac continued, "Here are the fire and the wood, but where is the sheep for the burnt offering?"
"My son," Abraham answered, "God will provide the sheep for the burnt offering."
The concept of sacrifice is evident from the very beginning of the Bible. A few details in particular stick out:
We pick back up in the book of Exodus for the Passover, when God frees His people from slavery in Egypt:
The LORD said to Moses and Aaron in the land of Egypt:
This month will stand at the head of your calendar; you will reckon it the first month of the year. Tell the whole community of Israel: On the tenth of this month every family must procure for itself a lamb, one apiece for each household. If a household is too small for a lamb, it along with its nearest neighbor will procure one, and apportion the lamb's cost in proportion to the number of persons, according to what each household consumes.
Your lamb must be a year-old male and without blemish. You may take it from either the sheep or the goats. You will keep it until the fourteenth day of this month, and then, with the whole community of Israel assembled, it will be slaughtered during the evening twilight. They will take some of its blood and apply it to the two doorposts and the lintel of the houses in which they eat it. They will consume its meat that same night, eating it roasted with unleavened bread and bitter herbs.
For on this same night I will go through Egypt, striking down every firstborn in the land, human being and beast alike, and executing judgment on all the gods of Egypt — I, the LORD! But for you the blood will mark the houses where you are. Seeing the blood, I will pass over you; thereby, when I strike the land of Egypt, no destructive blow will come upon you.
Moses summoned all the elders of Israel and said to them, "Go and procure lambs for your families, and slaughter the Passover victims. Then take a bunch of hyssop, and dipping it in the blood that is in the basin, apply some of this blood to the lintel and the two doorposts."
A few details to note:
In his prophecies about the Messiah, Isaiah likens Him again to a lamb:
Though harshly treated, he submitted and did not open his mouth; like a lamb led to slaughter or a sheep silent before shearers, he did not open his mouth.
Even John the Baptist aludes to the Lamb:
The next day [John] saw Jesus coming toward him and said, "Behold, the Lamb of God, who takes away the sin of the world. He is the one of whom I said, 'A man is coming after me who ranks ahead of me because he existed before me.'"
In the book of Revelation, we finally see the lamb triumphant:
Then I saw standing in the midst of the throne and the four living creatures and the elders a Lamb that seemed to have been slain.
[T]hey cried out in a loud voice: "Worthy is the Lamb that was slain to receive power and riches, wisdom and strength, honor and glory and blessing."
All of these prophecies come to a head when Jesus is on the cross:
After this, aware that everything was now finished, in order that the scripture might be fulfilled, Jesus said, "I thirst." There was a vessel filled with common wine. So they put a sponge soaked in wine on a sprig of hyssop and put it up to his mouth.
When Jesus had taken the wine, he said, "It is finished." And bowing his head, he handed over the spirit.
Now since it was preparation day, in order that the bodies might not remain on the cross on the sabbath, for the sabbath day of that week was a solemn one, the Jews asked Pilate that their legs be broken and they be taken down. So the soldiers came and broke the legs of the first and then of the other one who was crucified with Jesus. But when they came to Jesus and saw that he was already dead, they did not break his legs, but one soldier thrust his lance into his side, and immediately blood and water flowed out.
Details to note:
It is clear, then, that Jesus is the Lamb of God. To Jews, this would remind them of God's awesome power to save them. But this title is more than just figurative, as we'll soon see.
In the Gospel of John, Jesus is likened to the Word, which became flesh:
In the beginning was the Word, and the Word was with God, and the Word was God.
And the Word became flesh and made his dwelling among us
What does this mean? Remember that God (i.e., Jesus) is Truth. His words effect reality. To understand this, consider a baseball umpire.1 When the umpire calls a player out, his words do not simply describe reality, but in fact change reality. Whether the player was actually out or not, the umpire declaring him so means that he is out.2 In the same way, God's words, who are far more powerful, change reality. That is, reality changes to match God's words, since what He speaks cannot be false. Consider this line from the creation story:
In the beginning, when God created the heavens and the earth — and the earth was without form or shape, with darkness over the abyss and a mighty wind sweeping over the waters —
Then God said: Let there be light, and there was light.
and then a little later:
Then God said: Let the water under the sky be gathered into a single basin, so that the dry land may appear. And so it happened: the water under the sky was gathered into its basin, and the dry land appeared.
Notice that each day in the creation story follows the same format: God speaks, and reality conforms to what He says.
Jesus's words have the same power. Consider the myraid miracles He performs. Perhaps the best example is in Mark 5, where Jesus heals Jairus's daughter by commanding her to get up:
So he went in and said to them, "Why this commotion and weeping? The child is not dead but asleep." And they ridiculed him. Then he put them all out. He took along the child’s father and mother and those who were with him and entered the room where the child was.
He took the child by the hand and said to her, "Talitha koum," which means, "Little girl, I say to you, arise!" The girl, a child of twelve, arose immediately and walked around. [At that] they were utterly astounded.
We've seen, then, that Jesus's words change reality. This is true especially in John 6. After having multiplied the loaves and fishes, Jesus walks across the water to escape the crowds, but they find Him again. Jesus then begins what is known as the bread of life discourse. At the end of it, He concludes:
"I am the bread of life. Your ancestors ate the manna in the desert, but they died; this is the bread that comes down from heaven so that one may eat it and not die. I am the living bread that came down from heaven; whoever eats this bread will live forever; and the bread that I will give is my flesh for the life of the world."
The Jews quarreled among themselves, saying, "How can this man give us [his] flesh to eat?"
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 have 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. Whoever eats my flesh and drinks my blood remains in me and I in him. Just as the living Father sent me and I have life because of the Father, so also the one who feeds on me will have life because of me. This is the bread that came down from heaven. Unlike your ancestors who ate and still died, whoever eats this bread will live forever."
This is reminiscent of the unleavened bread eaten at Passover (above) and the manna in the desert.3 When Jesus finished, the Jews begin to leave Him:
Then many of his disciples who were listening said, "This saying is hard; who can accept it?"
As a result of this, many [of] his disciples returned to their former way of life and no longer accompanied him.
Note that this is the only place in the Bible where people stop following Jesus because of one of His teachings (John 6:66). Jesus's unwillingness to change his teaching underscores its importance, even at the loss of followers.
But we have to ask ourselves: what makes the Jews say that "This saying is hard"? If Jesus is speaking metaphorically, they should have had no trouble accepting what He said. This suggests that Jesus was speaking literally; that is, that the Jews would have to physically eat Jesus. The Greek text supports this: the words in Greek used to indicate "eat" are not the usual ones to indicate human eating; rather, they more accurately indicate "gnawing" or "chewing" or "munching", as animals eat. This would be a hard saying to accept: "How can this man give us [his] flesh to eat?", as the Jews put it. If Jesus was speaking metaphorically, He certainly would have corrected the Jews' mistaken impression that they'd have to eat his flesh. Once again, Jesus's unwillingness to change his teaching, even at the loss of followers, underscores the teaching's importance.
The Catholic Church takes Jesus at His Word, then, believing that at the words of consecration in Mass, the bread and wine become Jesus's body and blood. While the appearance of the bread and wine don't change (their "accidents"), the essential nature of them has become Jesus (their "substance"). To back up the Church's claims, many Eucharistic miracles have been witnessed (and in all fairness: plenty of purported miracles which were eventually ascribed to natural causes), whereby a consecrated host begins bleeding or visibly turns into human flesh.