The Catholic Encyclopedia
The word, though casually taken in Holy Writ in a metaphorical sense, is most generally used by the sacred writers to designate the first male child in a family. The first-cast male animal is, in the English Bibles, termed "firstling". The firstlings, both human and animal, being considered as the best representatives of the race, because its blood flows purest and strongest in them, were commonly believed, among the early nomad Semitic tribes, to belong to God in a special way. Hence, very likely, the custom of sacrificing the first-cast animals; hence also the prerogatives of the first-born son; hence, possibly, even some of the superstitious practices which mar a few pages of the history of Israel.
Among the Hebrews, as well as among other nations, the first-born enjoyed special privileges. Besides having a greater share in the paternal affection, he had everywhere the first place after his father (Genesis 43:33) and a kind of directive authority over his younger brothers (Genesis 37:21-22, 30, etc.); a special blessing was reserved to him at his father's death, and he succeeded him as the head of the family, receiving a double portion among his brothers (Deuteronomy 21:17). Moreover, the first-birthright, up to the time of the promulgation of the Law, included a right to the priesthood. Of course this latter privilege, as also the headship of the family, to which it was attached, continued in force only when brothers dwelt together in the same house; for; as soon as they made a family apart and separated, each one became the head and priest of his own house.
When God chose unto Himself the tribe of Levi to discharge the office of priesthood in Israel, He wished that His rights over the first-born should not thereby be forfeited. He enacted therefore that every first-born be redeemed, one month after his birth, for five sicles (Numbers 3:47; 18:15-16). This redemption tax, calculated also to remind the Israelites of the death inflicted upon the first-born of the Egyptians in punishment of Pharaoh's stubbornness (Exodus 13:15-16), went to the endowment-fund of the clergy. No law, however, stated that the first-born should be presented to the Temple. It seems, however, that after the Restoration parents usually took advantage of the mother's visit to the sanctuary to bring the child thither. This circumstance is recorded in St. Luke's Gospel, in reference to Christ (ii, 22-38). It might be noted here that St. Paul refers the title primogenitus to Christ (Hebrews 1:6), the "first-born" of the Father. The Messianic sacrifice was the first-fruits of the Atonement offered to God for man's redemption. It must be remembered, however, contrary to what is too often asserted and seems, indeed, intimated by the liturgical texts, that the "pair of turtle-doves, or two young pigeons" mentioned in this connexion, were offered for the purification of the mother, and not for the child. Nothing was especially prescribed with regard to the latter.
As polygamy was, at least in early times, in vogue among the Israelites, precise regulations were enacted to define who, among the children, should enjoy the legal right of primogeniture, and who were to be redeemed. The right of primogeniture belonged to the first male child born in the family, either of wife or concubine; the first child of any woman having a legal status in the family (wife or concubine) was to be redeemed, provided that child were a boy.
As the first-born, so were the firstlings of the Egyptians smitten by the sword of the destroying angel, whereas those of the Hebrews were spared. As a token of recognition, God declared that all firstlings belonged to Him (Exodus 13:2; Numbers 3:3). They accordingly should be immolated. In case of clean animals, as a calf, a lamb, or a kid (Numbers 18:15-18), they were, when one year old, brought to the sanctuary and offered in sacrifice; the blood was sprinkled at the foot of the altar, the fat burned, and the flesh belonged to the priests. Unclean animals, however, which could not be immolated to the Lord, were redeemed with money. Exception was made in the case of the firstling of the ass, which was to be redeemed with a sheep (Exodus 34:20) or its own price (Josephus, Ant. Jud., IV, iv, 4), or else to be slain (Exodus 13:13; 34:20) and buried in the ground. Firstlings sacrificed in the temple should be without blemish; such as were "lame or blind, or in any part disfigured or feeble", were to be eaten unconditionally within the gates of the owner's home-city.
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Obstat, Nihil. Lafort, Remy, Censor. Entry for 'First-Born'. The Catholic Encyclopedia. https://www.studylight.org/encyclopedias/eng/tce/f/first-born.html. Robert Appleton Company. New York. 1914.