The author challenges us to think of who Joseph was. Joseph did take Mary as his wife. He did parent Jesus. Jesus would have called Joseph, "Abba". Jesus obeys Joseph. And, Joseph would have been responsible according to Deuteronomy 6 to train Jesus in the scriptures.
And, "Jesus' identity as the Christ, after all, is tied to his identity as the ancestor of David, the legitimate heir to David's throne. Jesus saves us as David's son, the offspring of Abraham, the Christ. That human identity came to Jesus through adoption. Matthew and Luke trace Jesus' roots in abrham and David through the line of Joseph. As the Presbyterian scholar J. Gresham Machen put it, Joseph's adoption of Jesus means Jesus belongs 'to the house of David just as truly as if he were in a physical sense the son of Joseph. He was a gift of God to the Davidic house, not less truly, but on the contrary in a more wonderfu way than if he had been descended from david by ordinary generation.' It is through Joseph that Jesus find his identity as the fulfillment of the Old Testament promise. It is through Joseph's legal fatherhood of Jesus that the 'hopes and fears of all the years' find their realization in the final son of Abraham, son of David, and son of Israel. Joseph's fatherhood is significant for us precisely because of the way the gospel anchors it to the fatherhood of God himself." (page 67 & 68)
I have never considered that Jesus was adopted. That is amazing! I love it. I am amazed at how many ways Jesus can relate to us in our specific situations and sufferings. WoW!