God accepts the Levites as substitutes for the firstborn of Israel, who properly belong to God because they were spared in the exodus.
God’s right to the firstborn sons of Israel is anchored in the story of the exodus. Since God spared Israel’s firstborn, God says that they “shall be mine.” In some ancient religious practices, this might have meant human Sacrifice is commonly understood as the practice of offering or giving up something as a sign of worship, commitment, or obedience. In the Old Testament grain, wine, or animals are used as sacrifice. In some New Testament writings Jesus' death on the cross as the..., but God rejects that option and allows instead that the firstborn be “redeemed” or bought back (see Exodus 13:11-16; Numbers 18:15-16). Here in Numbers 3, God accepts the Levites, a tribe not included in the census for service for war or in the distribution of land, as “substitutes” for Israel’s firstborn (see also 3:40-51).This will be part of the basis for the Levitical priesthood.
The exodus narrative, picked up here in Numbers, gives a historical basis for the common offering to God of all first fruits (for example, Exodus 23:19; Numbers 18:13; Deuteronomy 18:4). Returning to God the first fruits of Creation, in biblical terms, is the universe as we know or perceive it. Genesis says that in the beginning God created the heavens and the earth. In the book of Revelation (which speaks of end times) the author declares that God created all things and... acknowledges that all belongs to God and is graciously given by God. Some have seen the story of Abraham’s near-sacrifice of Son born to Abraham and Sarah in fulfillment of God's promise as another indication that, though once people thought the sacrifice of a firstborn child was appropriate, God will instead “redeem” the child through another form of sacrifice (Genesis 22:1-19).