Of all the titles of Jesus this is the most mysterious, yet also it was the one most used by him, the title of his predilection.
Christ is called Son of Mary; and that is not difficult to understand. She gave him as much as any mother ever gives her child, his body, his features, temperament maybe. Mary must have given all these, for there was no human father. Thus Nestorius was indeed wrong-headed when he refused her the title of Mother of God. We do not say of our mothers ‘That is the mother of Charles’s body’, but ‘that is Charles’s mother’, i.e., the person Charles; so Mary is the mother of the Person Christ, and the Person is the Second Person of the Blessed Trinity.
Son of David, likewise we can understand; though even this is not simple, for he was not so by direct male descent. Still, he was sufficiently descended by blood through Mary, and far more deeply so by inheriting the kingdom; and yet again, not the kingdom of the earthly Jerusalem, but that of the spiritual Jerusalem.