Numbers 3:44

And the LORD spake unto Moses, saying,

The LORD speaking to Moses here gives the order for priestly and Levitical service. Every firstborn male child, like the first-fruits of the farm, belonged to the Lord and was consecrated to Him because the firstborn of Israel had been preserved from the destroyer who slew the firstborn of the Egyptians. Although the oldest sons would naturally have become the priests and religious teachers, God afterward chose the whole tribe of Levi for that service instead of the firstborn sons.

Even after the Levites were appointed to sanctuary service, Israel was still required to redeem its firstborn, with the firstborn son redeemed back to the family in grateful recognition by the payment of five shekels, about $2.75 to nearly three dollars, and the redemption proceeds went to the Levites. The chapter establishes the Aaronic priesthood by law for its appointed age, until the type met its antitype, restricting priestly service to Levi under that order. This spoken word stands as what is written in the book of Moses and supplied the pattern followed at the dedication of the rebuilt temple, when the priests were set in their divisions and the Levites in their courses for the service of God at Jerusalem.

This redemption of the firstborn, commonly called presentation in the temple, was performed for the child Jesus in the temple together with the purification offerings of the mother. The redemption teaches that parents ought to consecrate every child to God and ever keep him before the Lord with earnest prayer.