Agnus Dei is Latin for “Lamb of God.” The text is drawn from John 1:29, where John the Baptist points his disciples to Jesus and says, “Behold, the Lamb of God, who takes away the sin of the world!” By calling Jesus the Lamb of God, John calls to mind several passages from the Old Testament which find their fulfillment in Christ. On the Day of Atonement, one goat was sacrificed for the people while their sins were laid upon a second goat to be cast out into the wilderness as the scapegoat (Leviticus 16). Perhaps more prominently, the Passover lamb, which was to be a lamb without blemish, was sacrificed for the Passover meal and its blood sprinkled on the doorposts as a sign that death had already entered that house and so could not enter again (Exodus 12). Finally, the prophet Isaiah pointed to the Suffering Servant as a lamb led silently to the slaughter (Isaiah 53:6-7). Christ Jesus, the true Paschal Lamb, is the fulfillment of all the prophecies and sacrifices of the Old Testament—the one and only sacrifice for the sin of the world whose blood was poured out for many and is now given to us with His body to eat and to drink in the Lord’s Supper (Matthew 26:28).
The Agnus Dei made its way into the Western Church by the end of the seventh century and has remained associated with the Eucharist ever since. Having heard the Words of Our Lord spoken over the elements and the declaration that Christ Himself is our peace with God, the Church now joins together in song to confess that Christ, the Lamb of God, is truly present in the Sacrament. The Agnus Dei is a threefold prayer directed to Christ specifically as He is present on the altar under the bread and wine which we are about to receive.
In the Kyrie and Gloria in Excelsis earlier in the service, we sang of Christ’s salvific work and begged for God’s mercy to be shown to the world, the Church, and ourselves. That prayer now takes concrete shape as we plead for God’s mercy and peace to be given to us in Christ’s body and blood. It is only through God’s mercy, having been sprinkled with Christ’s blood and buried into death with Him in our Baptism (Romans 6:4), that we can approach the altar of God and live. The Agnus Dei connects us to the whole Church triumphant, the saints in heaven who sing the eternal song of praise, “Worthy is the Lamb who was slain” (Revelation 5:12). Heaven descends to earth in this foretaste of the feast to come as Christ, the Lamb of God who takes away the sin of the world, gives to us the peace which only He can give—the forgiveness of sins, eternal life, and salvation.