Daly, Robert J. "Part 2: From the Old Testament to the New. Chapter Six: The Pasch." Christian Sacrifice: The Judaeo-Christian Background Before Origen. Washington, D.C.: The Catholic University of America Press, 1978, 196-207.
Daly notes that the Pasch is not only considered the greatest of the Jewish feasts but that Origen in particular considered the identification of Jesus as our Pasch (1 Cor. 5:7) as an essential motif in Christian thought (Daly 1978, 196).
The sacrificial elements of the Pasch are undeniable. However, among the sacrifices, the Pasch was unique due to the sacred meal which accompanied the sacrifice (Daly 1978, 197). Daly identifies three different stages in the concept of the Pasch: first an Egyptian deliverance from Exodus 12, then a feast from Deuteronlmy 16 centered around temple observance, then a Jewish compromise of the former two, practiced at the time of Jesus (Daly 1978, 198).
Daly finds in the Pasch as celebrated at the time of Jesus and re-cast in the New Testament as the Last Suipper, three important time elements. As a remembrance of the past, it commemorates the Exodus (Daly 1978, 199). Those eating become participants in God's rescue of His people from Egypt. In the sacrificial aspect of the meal, Israelites confessed that God was present with them and for them, thus creating a present dimension of the celebrations (Daly 1978, 201). The future element is also present as the partakers look forward to their final salvation, an event which they gradually came to associate with a future Passover meal (Daly 1978, 202).
The lamb of the Pasch was to be without blemish. Daly observes that in Exodus 12 and 2 Chronicles 35 a kid goat was equally acceptable and that Deuteronomy 16 includes cattle (Daly 1978, 203). However, the lamb became the commonly used animal.
The lamb became associated with Christ due to the Old Testament idea of redemption of the first born through a sacrifice. Jesus as the "first born" was thus readily associated with the lamb. Though the idea of a substitutionary atonement or a penal sacrifice may not have been present in the time of the Exodus, it was certainly present by the time of Jesus (Daly 1978, 205). The New Testament descriptions of Jesus as the Passover lamb were easily derived from the Paschal observances of the time (Daly 1978, 206).