1 And the Lord spake unto Moses, saying,

2 Sanctify unto me all the firstborn, whatsoever openeth the womb among the children of Israel, both of man and of beast: it is mine.

God had spared the lives of all the Israelite firstborn, but that didn’t mean He was leaving them untouched. Just as the Israelites had marked their doors with the blood of the lamb, God had marked all of Israel’s firstborn sons to serve Him.

Notice that God says to “sanctify” the firstborn, not to “sacrifice” them. These boys would not be killed like the Egyptians, but they would still be giving up their lives in the service of God. They would be the priests to carry out the rituals of the temple, performing sacrifice and making atonement for the masses. And just as every family in Egypt was touched by the death of their firstborn, so too every family in Israel would be linked to God, also through their firstborn. Every Israelite would personally know someone who was part of this sanctified work, the firstborn would be the leaven that raised the whole nation.

Of course, this arrangement would only persist for a season. In time, the tribe of Levi would prove itself more faithful, and as a reward God would assign them to be the new priest class. All other tribes would retain their firstborn sons, then depending on the righteousness of more pure families to keep their connection to God.