In the patriarchal order, the firstborn son is the heir and inherits the leadership of the family upon the death of the father. This is often spoken of in the scriptures as birthright (Gen. 43:33). Under the law of Moses, the firstborn son was regarded as belonging to God, and special ordinances were provided for his redemption (Ex. 13:12–15; 22:29; Num. 3:45; Luke 2:22–23). The eldest son received a double portion of his father’s possessions (Deut. 21:17); and after his father’s death, he was responsible for the care of the mother and sisters. The male firstborn of animals also belonged to God. Clean animals were used for sacrifices, while unclean animals might be redeemed, sold, or put to death (Ex. 13:2–13; 34:19–20; Lev. 27:11–13, 26–27).
Jesus is the firstborn of the spirit children of our Heavenly Father, the Only Begotten of the Father in the flesh, and the first to rise from the dead in the Resurrection, “that in all things he might have the preeminence” (Col. 1:13–18).
The faithful saints were made members of the Church of the Firstborn in eternity, since they receive through Jesus Christ the inheritance of the firstborn (D&C 93:21–22). Joseph, though not the firstborn of Jacob, received the inheritance as though he were the firstborn (1 Chr. 5:1–2), as also did Ephraim (Gen. 48; Jer. 31:9).
In matters of priesthood, in the Aaronic order, certain prerogatives descend upon the firstborn. For example, the right of presidency, to hold the keys of that priesthood, pertains to the firstborn in the family of Aaron (D&C 68:16–21).