Numbers 18:16
And those that are to be redeemed from a month old shalt thou redeem, according to thine estimation, for the money of five shekels, after the shekel of the sanctuary, which is twenty gerahs. And those that are to be redeemed from a month old shalt thou redeem, according to thine estimation, for the money of five shekels, after the shekel of the sanctuary, which is twenty gerahs.
After the tribe of Levi was set apart to serve in place of Israel's firstborn, every firstborn son was still to be redeemed by a ransom payment instead of serving in the sanctuary. That payment acknowledged God's mercy in preserving Israel's firstborn from the judgment that struck Egypt. The law requiring the presenting of the firstborn thus recalled Israel's deliverance from Egypt. At the same time it pointed beyond that deliverance to a greater deliverance, for as the lamb's blood spared Israel's firstborn, Christ's blood has power to save the world.