Question: Why did You promise them a redeemer?
God’s answer:
At this point, God led me to the pidyon haben, “Redemption of the firstborn so”’, a ceremony in Judaism wherein the father of a firstborn male redeems his son by giving a priest (kohen) five silver coins.3
“This practice came about because originally, the firstborn son was to be the priest (kohen) of the Jewish family. After the Exodus from Egypt, the Israelites committed the Sin of the Golden Calf, except for the tribe of Levi. As a result, the firstborn sons of the non-Levites were disqualified.
The LORD decreed that the Levites were to take the place of the firstborn sons of Israel, and required that all firstborn sons (who were not themselves Levites or Kohens) must be redeemed from service to God by means of paying five shekels of silver to the Levites.”4
Likewise, Adam and Eve are the first created children of God, both consecrated to Him. Just as he commanded the Israelites to redeem their first born son, so must He.
4 Hebrew4christians.com, 'Pidyon Haben' (2015) <http://www.hebrew4christians.com/Life_Cycle/Pidyon_Haben/pidyon_haben.html> accessed 25 August 2015.