"you shall say, 'It is the sacrifice of the Lord 's Passover, for he passed over the houses of the people of Israel in Egypt, when he struck the Egyptians but spared our houses.'" -Exodus 12:27
"Agnus Dei" - Worthy is the Lamb - Third Day
The Biblical significance of the title is rendered in the context of earlier lamb symbolism in addition to the literal reference to Jesus in chapter 53.
The blood of the paschal lamb of the Old Testament protects and saves the Israelites in Exodus 12. This link is made explicit in 1 Corinthians 5:7. For Paul, Christians are saved by Christ as their true paschal lamb.
The Hebrew Bible also testifies to the earlier practice of sin offerings as a possible means of atonement. Lambs could be used in these offerings (e.g. Leviticus 4:32-34 and 5:6), and this link is strongly suggested by John 1:29 and 1 Peter 1:19. Like the sin of a person could be forgiven through the offering and the pouring out of the blood of an "unblemished" lamb (cf. Lev 4:32), so Christians would be freed from sin by the blood of Jesus as the unblemished Lamb of God. See Sin for further discussion about the concept of sin and the means of atonement in Judaism.
It's all about Jesus... (Isaiah 53 ESV)... written between 701 and 681 B.C. approximately 700 years before the birth of Jesus.