On the nature of the Passover as penal substitution, from Pierced for Our Transgressions: Rediscovering the Glory of Penal Substitution
The first nine plagues present no danger to the Israelites. God’s judgment falls only upon the Egyptians, for as we saw above, God makes a distinction between them and his people. What is perhaps a little surprising is that in the tenth plague this distinction between Israel and Egypt is conditional. The firstborn of the Israelites are not automatically spared from death; a lamb must be slaughtered, and its blood applied to the door frame of the house. The clear implication is that the firstborn son of the Israelite families would die if this instruction were not followed, for the Lord had said, ‘when I see the blood, I will pass over you’ (Exod. 12:13; italics added). Thus the lamb becomes a substitute for the firstborn son, dying in his place.
…It is not only the firstborn sons who are involved in the Passover, however. The fact that the whole family shares together in the symbolic meal implies a wider application. Indeed, the striking emphasis on the proportionality between the amount of meat needed and the size of the Israelite household is between the amount of the meat needed and the size of the Israelite household is most likely intended to highlight this: ‘If any household is too small for a whole lamb, they must share one with their nearest neighbour, having taken into account the number of people there are. You are to determine the amount of lamb needed in accordance with what each person will eat’ (Exod. 12:4). This extended comment would seem somewhat superfluous if intended only as a piece of culinary etiquette to guard against wasting food. An enslaved people, about to flee on a long and arduous journey into the desert, would hardly need to be warned against that!
The substitutionary element in the Passover is therefore beyond dispute. Moreover, given that the plagues function unambiguously as instruments of divine judgment, penal substitution is plainly taught here. This might seem puzzling, for while it is obvious why God would decide to punish the Egyptians, why would he judge his people? This seems all the more surprising given that the plague on the firstborn is described specifically as ‘judgment on all the gods of Egypt’ (Exod. 12:12). According to Ezekiel 20:4-10, however, the Israelites participated in the idolatry of their Egyptian masters; they too were guilty, and were no less deserving of God’s judgment. Only by God’s gracious provision of a means of atonement, a substitutionary sacrifice, were they spared.
We learn in the New Testament that Christ’s death is the fulfillment of the Passover. Or we might say that the Passover stands as the background against which we understand the death of Christ. Christ “suffered in the place of his people in order that they might be marked out by his blood and thus spared from God’s wrath.”