13 And the blood shall be to you for a token upon the houses where ye are: and when I see the blood, I will pass over you, and the plague shall not be upon you to destroy you, when I smite the land of Egypt.
Original sin could not have easily been forgiven, if a victim had not been offered for it, if that sacred blood of propitiation had not been shed. Even then the words in Exodus were not vainly said of our Lord: “I shall see the blood and shall protect you.” That figure of the lamb represented this passion of Christ our Lord. Blood is given for blood, death for death, a victim for sin, and thus the devil lost what he held.
[AD 542] Caesarius of Arles on Exodus 12:13