Of the line of David?

After church today, someone asked me why we say that Jesus is “of the line of David” when Joseph was his step-father. It was an insightful question, one that inattentive readers of the Bible often fail to even raise.

The irony, I replied, is that two incompatible genealogies for Jesus are preserved in precisely those Gospels that make Joseph’s lineage irrelevant. In both, it is explicitly Joseph’s lineage that is given, and never Mary’s. Once again, fundamentalist sometimes claim that one of the genealogies belongs to Mary because it is more important to be able to say that the Bible is right than to actually pay attention to what the Bible says.

The easiest explanation (and just one of the ones I mentioned in response to this question) is that Jesus was Joseph’s son, and born into a family with the reputation of being descended from David. The later stories of miraculous conceptions were expressions in the appropriate way for that time of the importance of Jesus, and the conviction that he must be the Son of God in a sense that no one else was.

This is not to say that the conservative explanation, that Joseph’s acceptance of Jesus as his son would have made him legally of the line of David, is necessarily untrue. But it remains the case that Paul did not require such convoluted explanations. He didn’t know stories of Jesus’ miraculous conception. He believed Jesus was descended from David according to the flesh.

My own main point in answering was that the very notion that such questions have a single, simple answer is itself wide of the mark. How would you have answered this question?