We are saved by the Blood of the Lamb

God sent Moses to Pharaoh with the message to let God’s people leave Egypt. When Pharaoh refused, God sent plague after plague upon Egypt, nine in all, but Pharaoh would not allow the Hebrews to leave Egypt. Finally, God sent a tenth plague, the death of the first born in each household. God told Moses that the angel sent to bring destruction would pass-over the homes of the Hebrews if they followed two commandments.
First, each home must select a lamb and sacrifice it. Then they were to put the blood of the lamb on the woods of their doors: on the doorposts and on the lintel. God promised that when the angel sent to strike death saw the blood on the wood of their doors, the angel would pass-over those homes when He struck down Egypt. To this day, our Jewish neighbors celebrate this feast of Passover.
When Jesus was about 30 years old He began His public ministry. The first thing He did was to go down to the Jordan River where His cousin, John, was baptizing. When John saw Jesus coming toward him, he pointed Jesus out to the crowd and said: “Behold, the Lamb of God who takes away the sin of the world!” Of all the things which John could have said (Behold the Messiah, or the teacher, or the promised one, or Jesus, etc.) he called Jesus an animal, the Lamb of God. 
When we 21st century Americans hear the word “lamb” we may think of a cute, little animal which we may want to pet. This would not have been true of the people at the time of Jesus. The title Lamb of God was not cute. It meant something to be sacrificed, to be killed, not petted. Inspired by the Holy Spirit, John introduces the One who was to shed His blood for us.
At the Last Supper, Jesus is celebrating the Passover. In effect, He tells His apostles that He is the true Lamb of God and that the next day, when His Father would see His blood, not on the wood of their doors but on the cross, God’s judgment would pass-over them and sin may be forgiven. Finally, they will be free to go to eternal life, the true Promised Land. We are saved by the Blood of the cross, the Blood of the Lamb.
But let’s return to the first Passover in Egypt. As I stated in the first paragraph above, God gave Moses two commandments that night of the first Passover. The first was the sacrifice of the lamb. The second was this: that the lamb must be eaten. There are many sacrifices mentioned in the Bible, but that did not mean that they were to be eaten. Often the sacrifices were burned. But the Passover lamb must be eaten.
Since Jesus was the true Lamb of God, He must be eaten. At the Last Supper Jesus took the bread and told the apostles, “This is my Body.” He likewise took the wine and said, “This is my Blood.” “Take and eat,” He said; “Take and drink.” He fulfilled the Passover by being the sacrifice whose blood was placed upon the wood, and being the sacrifice who was to be eaten. He is our Passover which brings us from slavery to sin to the salvation God offers us. 
As St. John Chrysostom wrote centuries ago: “In those days, when the destroying angel saw the blood on the doors he did not dare to enter, so how much less will the devil approach now when he sees, not that figurative blood on the doors, but the true Blood on the lips of believers, the doors of the temple of Christ.” 
At every Mass the priest, immediately before Communion, raises the Eucharist and repeats the words of John: “Behold the Lamb of God who takes away the sin of the world!” We are called to believe in the Lamb whose Blood was shed for us and whose Body and Blood now nourish us on our journey to the life He has won for us on the cross.