In Galatians 4, Paul expounds on the doctrine of adoption. That is, the doctrine that, even though we were born into sin and death, God sent his Son Jesus to redeem those under this curse of sin and death, so that we could be adopted into God’s family. Paul puts it like this:
In the same way we also, when we were children, were enslaved to the elementary principles of the world. But when the fullness of time had come, God sent forth his Son, born of a woman, born under the law, to redeem those who were under the law, so that we might receive adoption as sons. And because you are sons God has sent the Spirit of his Son into our hearts, crying “Abba! Father! So you are no longer a slave, but a son, and if a son, an heir through God. (Galatians 4:3-6)
In other words, because God sent forth his Son, we are no longer slaves, but sons and heirs through God. We have been adopted into God’s family!
In the all important genealogy of Jesus in Matthew 1, Jesus is shown to be a descendant of Abraham, Isaac, Jacob, David etc . . . all the way down to Joseph.
Abraham was the father of Isaac, and Isaac the father of Jacob, and Jacob the father of Judah and his brothers . . . and Jacob the father of Joseph, the husband of Mary, of whom Jesus was born, who is called Christ.” (Matthew 1:2, 16)
Blink and you miss it. Joseph was Mary’s husband. Jesus was born to Mary. Mary conceived by the power of the Holy Spirit (Luke:1:34-35). Jesus is no blood relation of Joseph at all. There is no genetic connection to David in his birth. Jesus is born into the line of David by virtue of his adoptive father.
Jesus is adopted into the royal bloodline; the Son of God, adopted into the family of David. We are adopted into a royal bloodline also; we are Sons of Abraham, adopted into the family of God. Jesus models adoption, and then adopts us by his atoning death, redeeming those under the law, “that we might receive adoption as sons.”