Numbers 3:13
Because all the firstborn are mine; for on the day that I smote all the firstborn in the land of Egypt I hallowed unto me all the firstborn in Israel, both man and beast: mine shall they be: I am the LORD. Because all the firstborn are mine; for on the day that I smote all the firstborn in the land of Egypt I hallowed unto me all the firstborn in Israel, both man and beast: mine shall they be: I am the LORD.
On the day the firstborn of Egypt were struck, the firstborn of Israel were spared, and for that reason God claimed them as his own. That claim covered both man and beast. He hallowed them and set them apart as holy to himself.
Though preserved, Israel's firstborn had been liable to the same judgment as Egypt's, so their preservation acknowledged the atoning sacrifice. Their dedication was therefore appointed as a memorial commemorating Israel's deliverance. From the earliest times each household was to consecrate its firstborn son to God, devoting him to the priesthood as a representative of Christ.
The priestly office that formerly belonged to the firstborn of each family was later given to Aaron and his household. When sanctuary service began, God accepted the tribe of Levi for that service in place of Israel's firstborn. He chose Levi to honor their faithfulness to him and their carrying out of his judgments during the apostasy over the golden calf.
Even with the Levites serving in their place, the firstborn still belonged to God, and each had to be redeemed by a ransom price. Their dedication pointed forward to a greater deliverance through God's only-begotten Son. As the lamb's blood on the doorposts saved Israel's firstborn, Christ's blood alone has power to save the world. The dedication acknowledged God's gift of heaven's First-born given to save sinners.