Q. How does the genealogy of Christ work? Because if this is recorded in a patriarchal society, this is the line of Joseph, right? Doesn’t that mean none of this genealogy actually flows through Jesus’s blood? How is he from the line of King David if his real father is God and not Joseph?
The purpose of the genealogy in Matthew’s gospel is to demonstrate that Jesus is “the son of David, the son of Abraham,” that is, the legal heir of both of these men and thus the beneficiary (and ultimate fulfillment) of the covenant promises that God made to them.
All Jews were descended from Abraham. But Jesus was not descended from David, who was from the tribe of Judah, through his mother Mary, because she was instead a descendant of Aaron from the tribe of Levi. We know this because Luke’s gospel tells us that Mary was a “relative” of Elizabeth, who was a “descendant of Aaron.”
But when Joseph, who was descended from David, married Mary, this also constituted his legal adoption of the son she would bear. The language of Matthew’s genealogy reflects this legal understanding: “Joseph, the husband of Mary . . . the mother of Jesus.”
Later in Matthew’s gospel we see from the narrative that Jesus was considered to be Joseph’s son just as much as the other children that Mary and Joseph had together. The people of Nazareth ask, after Jesus tells a series of parables, “Isn’t this the carpenter’s son? Isn’t his mother’s name Mary, and aren’t his brothers James, Joseph, Simon and Judas? Aren’t all his sisters with us? Where then did this man get all these things?”
Accordingly Paul can say of Jesus at the beginning of his letter to the Romans, “who as to his earthly life was a descendant of David, and who through the Spirit of holiness was appointed the Son of God in power by his resurrection from the dead.”
Luke says similarly in his genealogy that Jesus was “thought” or “supposed” to be the son of Joseph; the International Standard Version says that he was “legally calculated” to be Joseph’s son, and I think that’s a good way of expressing the meaning here.
So Jesus was the son of Joseph in the full legal sense, because he was adopted when Joseph married Mary, and thus Jesus is also considered to be a legal descendant of David.