In many societies the firstborn child enjoys special privileges and responsibilities.
In Israel the firstborn (oldest son) received a double inheritance of every other son and a special blessing as his birthright (Deut 21:17). Esau didn’t value his birthright and sold it to Jacob for a meal, later sorely regretting it (Gen 25:29-34, 27:34).
The tenth and final plague that God sent on the Egyptians because Pharaoh would not allow the Israelites go, was the death of the firstborn sons and firstborn cattle (Ex 11:1-6). In contrast, because the Israelites 'protected' their homes by blood the angel of death would not touch them – this became known as the Passover (Ex 12:6,7,13,14). From the time of the Exodus the Israelites firstborn males (human and animal) were to be consecrated to God – the sons and some animals being redeemed (bought back by money) while other specific types of animals were sacrificed (Ex 13:2,12-16, 22:29,30, 34:19,20; Num 18:15,16).
In the absence of the father, the firstborn son had authority over his siblings. Female humans were seldom given any recognition or status in society until towards the end of the NT.
Christ's entry into His earthly ministry was as the firstborn of His mother (Lk 2:7). His taking on human likeness is termed the incarnation as the invisible God became visible in Christ; the divine clothed with humanity (Mt 17:2; Jn 1:14).
However, Jesus as part of the triune Godhead (Father, Son and Holy Spirit) has always existed and was the creator (Jn 1:1-4,10; Heb 1:2-4). As the eternal Son of God, He could not be both creator and created. "And He is the image of the nvisible God, the firstborn of all creation. For by Him all things were created (Col 1:15-20). Jesus is called the firstborn (not the first-created), and in the original Greek this signifies priority and rank, not necessarily the oldest child and so expresses Christ's sovereignty over creation. Jesus created the world, died to save the world, and rules the world. He also is the firstborn Son (among many redeemed Christians) of His heavenly Father so is in the position of honour and authority over us (Rom 8:29; Col 1:18; Heb 12:23; Rev 1:5).
See also: birthright, Esau, incarnation, Passover.