Luke 2:7 And she brought forth her firstborn son, and wrapped him in swaddling clothes, and laid him in a manger; because there was no room for them in the inn.
The firstborn son was significant to Israel. The Lord told Israel to sanctify and set apart the firstborn (Exodus 13). Yahweh instructed Israel the firstborn sons of animals/livestock and the firstborn sons of Israel were to be set apart. Yahweh is the Creator and Master of the Universe, he declared Israel as his own special people, this meant Yahweh claimed every firstborn son that passed through the matrix of any womb. Period.
Jesus was the firstborn son of Mary. According to Exodus 13, Yahweh already had dibs on Jesus. Jesus, however, was a firstborn in more than once sense–Jesus was unique and set apart trifecta.
1) Jesus — Firstborn and set apart because he is the firstborn son of Joseph and Mary
First, Jesus was set apart because he was the firstborn son of Mary, a faithful and holy daughter of Israel. The angel came to Mary and told her that the Spirit of the Lord would be upon her and that she would bear a child, the Christ, the hope and Salvation of Israel. This child was her firstborn, Jesus the Christ.
2) Jesus — Firstborn and set apart because he is the only begotten Son of God
Second, Jesus was set apart because he was the only begotten Son of the Father (Psalm 2:7; Hebrews 1:5; 1 John 4:9). As the Creed says, Jesus the Son of God is “God of God – Light of Light – very God of very God – begotten, not made – being of one substance with the Father – by whom all things were made.” The author of Hebrews emphasizes the uniqueness of Jesus, the second person of the Trinity, the Incarnate God-man, whom the Father set apart, when he rhetorically asks,
For unto which of the angels said he [God the Father] at any time, Thou art my Son, this day have I begotten thee? And again, I will be to him a Father, and he shall be to me a Son?
3) Jesus — Firstborn and is set apart because he is the Lord’s “Christ” (the promised Messiah)
And third, Jesus was set apart because he was the Christ, the foretold Messiah and Anointed One. After his birth, Joseph and Mary took the infant Jesus to the Temple to present him before the Lord. At the Temple they were greeted by a man who had the Holy Ghost upon him. That man was Simeon, to whom God had revealed:
That he should not see death, before he had seen the Lord’s Christ (Luke 2:26).
Jesus, the son of Joseph and Mary, was the Promised Christ, the Messiah. Simeon glorified God that he saw the Christ before his death. This child was the promised hope and salvation of Israel. The Angel of the Lord told Mary that her child was going to be the Son of the Highest, and that the Lord would place him as firstborn upon the Throne of David.
And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS. He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David: And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end (Luke 1:31-33).
The Lord placed the shepherd boy David on the Throne of Israel. He was not a firstborn, nor was he even one of Saul’s sons. However, the Lord appointed him to the position of firstborn and ruler of Israel. The Lord rejected wicked Saul (1 Samuel 16) and appointed David to be King.
Jesus Christ was not the firstborn. Adam was the first man. But God appointed Christ to be the Firstborn, the Second Adam and King. The Lord rejected wicked Adam because he rebelled against Yahweh. When Adam fell and was cursed with death, all of mankind died in Adam. We are, however, made alive in Jesus, who is appointed firstborn and set apart as our Christ, our salvation (1 Corinthians 15:22).
It pleased God the Father to ransom sinners by the life, death, and resurrection of His only begotten Son, the incarnate Jesus Christ, who was the firstborn of Mary. Christ was set apart and sanctified. If you are in Christ, then you are empowered by the work of the Holy Spirit and are also set apart and sanctified. Not because of anything we have done, but solely because Jesus Christ is the sanctified firstborn.