This is new content in the second edition of Praying the Mass: The Prayers of the People, Chapter 11, "Communion Rite".
We first called Christ the “Lamb of God” in the Gloria. This title is deeply significant and ancient in its origin, reaching back to Abraham, the first of the Patriarchs of Israel. God tested Abraham, asking him to offer his only son Isaac as a sacrifice. Isaac, bearing the wood for the sacrifice on his back as he and Abraham walked up the mountain, asked his father where the lamb to be offered was; Abraham replied “God will provide himself the lamb.” (Gen. 22:8) What Abraham found was no lamb, but a ram with its horns caught in a thicket. From that time on, God’s people sought the “lamb of God.”
To commemorate His liberating the Hebrews from
, God instituted the feast of Passover at which every family was to take a pure, unblemished lamb and kill it, marking their doorposts with its blood and roasting and eating its flesh. (cf. Ex. 12) In time, Egypt Jerusalem maintained a sacrificial flock of lambs so that families on pilgrimage for the Passover could be sure to have a pure and unblemished lamb (rather than bringing one with them on the journey). Still, these lambs were not God’s lamb, but each family’s lambs. Temple
It was not until
the Baptist cried out, “Behold, the Lamb of God!” (John 1:29, 36) that the fulfillment of this ancient promise was made known. This title, which might sound peculiar and obscure to our modern ears, would have rung loud and clear in the ears of the Jews to whom St. John spoke. And he was not the only one to make such a clear connection between Christ and the Passover lamb. St. John , writing to the Corinthians, announced that “Christ, our paschal lamb, has been sacrificed.” (1 Cor. 5:7) St. Paul St. Peter wrote that we have been ransomed “with the precious blood of Christ, like that of a lamb without blemish or spot.” (1 Pet. 1:19) And the Apostle and Evangelist, in his book of Revelation, refers to Jesus as “the Lamb” nearly thirty times, most notably describing His appearance as “a Lamb standing, as though it had been slain.” (Rev. 5:6) St. John
It is this Lamb of God, Jesus Christ, Who “takes away the sin of the world” by His self-sacrifice. (John 1:29) Every time we say that Jesus “take[s] away the sins of the world,” we should be aware that we are speaking in the present tense, not the past tense. This is a reminder that Christ’s work of redemption did not conclude with His death, but is continual and ongoing: the Mass is offered for the expiation of our sins.
 This scene is rich in Christological symbolism: Christ, like Isaac, bore the wood for His own sacrifice on His back, and He wore a crown of thorns, prefigured by the ram whose horns were caught in a thorn-bush.
 The ritual of the Passover also points to Christ: He, like the lamb, is pure and without blemish, and His bones were not broken when He was sacrificed.
 “Paschal” (as in “Paschal mystery”) comes from the Greek word pascha, which comes from the Hebrew word pesach, which means “Passover.”