The Agnus Dei represents the risen Christ who triumphs over death. This symbol is rich in significance. John the Baptist proclaimed Jesus to be the Lamb of God who takes away the sin of the world. In the Revelation, Jesus is portrayed as a lamb. Even in the Old Testament, God's provision of a ram as a substitute sacrifice for Isaac is an important type of Christ.

John 1:29 

The next day John saw Jesus coming toward him and said, "Look, the Lamb of God, who takes away the sin of the world!