Commentaries:
This relationship between the firstborn and the Feast of Unleavened Bread is repeated later in Exodus 34:18-20.
Staff
The Law of the Firstborn
We can safely conclude that the price of buying the Israelites' freedom was the devastation of Egypt's land, and above all, the killing of Egypt's firstborn. God designed the redemption of Israel's firstborn to remind them of the high cost of their liberty. The Egyptians slain for Israel's release belonged to God just as surely as the Israelites, but God used them to pay for Israel's freedom. That collective sacrifice became a type of Christ. The practical inference is that Israel was obligated to the One who paid the price—God. To us, that God would use virtually an entire nation to pay for another nation's freedom can be a stunning, even shocking concept. However, God is Creator. He owns everything and is certainly free to do as He pleases.
God will even things out later, though, as Isaiah 19:18-25 shows. Then, Egypt will once again be a great nation. The redeeming of Israel's firstborn was to serve as a costly and constant reminder that freedom is not free and that they were obligated to God for their redemption from Egypt. Forgetfulness produces ingratitude, which in turn produces disobedience because such people are no longer motivated by a sense of obligation to the One who worked so powerfully in their behalf (Deuteronomy 8:10-20).
John W. Ritenbaugh
The Elements of Motivation (Part Four): Obligation