WHEN MOSES CAME with the message, "Let my people go," he told Pharaoh that the LORD made a distinction (pedut) between Israel and the other nations (Ex. 8:23). When the first Passover was instituted, this distinction was expressed as the "ransom price" of the death of the firstborn of Egypt, both man and beast (Ex. 4:23; 12:29, Isa. 43:3). Consequently the Exodus was to be commemorated by the Jewish people through the consecration of the firstborn: All firstborn sons of Israel were called to be God's priests, responsible for offering sacrifices on behalf of their families (Ex. 13:2,12).
After the Exodus from Egypt, however, the Israelites committed the grievous sin of the Golden Calf, of which only the tribe of Levi was not guilty. Consequently the LORD decreed that the Levites were to take the place of the firstborn sons as priests of Israel (Num. 3:12-13). This substitution was represented by means of Pidyon ha-ben (the "redemption of the firstborn"), when a father would "redeem" his son by giving five shekels of silver to a priest a month after his son's birth (Num. 18:16, Ex. 13:13).