The meaning of First-born, Redemption of in the Bible
(From Easton's Bible Dictionary)

From the beginning the office of the priesthood in each family belonged to the eldest son. But when the extensive plan of sacrificial worship was introduced, requiring a company of men to be exclusively devoted to this ministry, the primitive office of the first-born was superseded by that of the Levites (Numbers 3:11-13), and it was ordained that the first-born of man and of unclean animals should henceforth be redeemed (18:15).

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