|The priest holds up the consecrated host during the Ecce Agnus Dei|
"For us, the light is Jesus Christ. All the Church is oriented, facing East, toward Christ: ad Dominum." —"Cardinal Sarah: ‘How to Put God Back at the Center of the Liturgy’ at the National Catholic RegisterThe fourth turn of the priest away from the altar during Mass occurs at the prayer "Ecce Agnus Dei." The priest recites this prayer when displaying the consecrated Host to the people before giving them Holy Communion. He says: "Ecce Agnus Dei, ecce qui tollit peccata mundi. Beati qui ad cenam Agni vocati sunt" ("Behold the Lamb of God, behold him who takes away the sins of the world. Blessed are those called to the supper of the Lamb").
Why is Christ called the Lamb of God? Christ as the Passover Lamb is prophesied in the Old Testament and clearly identified in the New.
“Though he was harshly treated, he submitted and opened not his mouth; like a lamb led to the slaughter or a sheep before the shearer, he was silent and opened not his mouth” (Isaiah 53:7).
The Torah instructed the Jews to observe the Feast of Passover, to recall the deliverance of the Jews from slavery in Egypt. Every year at the Passover feast, Jews ate a "lamb without blemish," to remind them of how the night before they fled Egypt, the Jews were instructed to sacrifice and eat a lamb and paint its blood over their doors. When God sent angels that night to destroy the first-born of the Egyptians, in punishment for the Pharoah's refusal to let His people go, the angels spared (passed over) His people who were protected by the blood of the Pascal Lamb.
In the New Testament, Christ is called the Lamb of God by St. John the Baptist in the Gospel of John the Evangelist, and He is revealed as the triumphant Lamb in heaven in the Book of Revelation. Christ instituted the Eucharist during His Last Supper, which was a Passover meal. John the Baptist saw Jesus coming to him, and he said: "Behold the Lamb of God, behold him who taketh away the sin of the world."
St. Peter wrote, “Realize that you were delivered from the futile way of life your fathers handed on to you, not by any diminishable sum of silver or gold, but by Christ’s blood beyond all price, the blood of a spotless, unblemished lamb…”
This closing quote from Catholic Straight Answers is from an excellent article that provides many rich details to more-completely answer the question "Why is Jesus called the 'Lamb of God'?”
"The Book of Revelation highlights this notion picturing the Lamb surrounded by angels, the “living creatures,” and elders, who cried out, “Worthy is the Lamb that was slain to receive power and riches, wisdom and strength, honor and glory and praise!” (Revelation 5:12). Jesus is the King of kings, and Lord of lords (Revelation 17:14) who will be victorious against the powers of evil and will invite the righteous to the wedding feast of the Lamb (Revelation 19:9), the union of the Church, the new Jerusalem, in heaven with the Lord."