In ancient Jewish sacrificial practices as set forth in the Torah, especially around the high holy days of Yom Kippur and Passover, the sacrifice of a lamb was highly symbolic. The earliest followers of Jesus, as they attempted to explain his significance for their lives after his death, used the language and symbols that they were familiar with to give expression to what they were currently coming to believe. Since the crucifixion of Jesus was such a traumatic and final moment, the people came up with a powerful way to give that death new meaning and significance.
Using the language and imagery of Yom Kippur and Passover gave the followers of Jesus not only the language, but the theology for what they experienced at the crucifixion. Look at the precedent. The sacrificial lamb had to be without blemish or defect in order for sins to be forgiven. This is exactly what the earliest followers of Jesus came to believe about the meaning of his death, hence the use of that language and imagery.
Gracious God, thank you for the ancient traditions which give our faith meaning. Amen. — RCW