Every now and then, the scriptures contain a hint of something that leads to a “wouldn’t it be nice if” supposition. Sometimes, the supposition has a lot of historical support and can be found in the early Church tradition or the exegesis of the Church Fathers. At other times, there’s no support for it at all.
One example of this is in the lineage of Mary. It is widely believed that Mary, like Joseph, was descended from the line of David, giving Jesus not just a legal claim to the royal lineage, but a natural one as well. The Gospel of Luke, however, contains the detail that
Jesus was (is) a divine person with a divine nature and a human nature. He was also the prophet, priest, and king par excellence. In his divinity, he possessed all of the qualities necessary for this triple role from eternity. In his humanity, however, these roles would need to be acquired according to human tradition.
The royal claim is clearly established through Matthew’s genealogical map from Abraham through David to Joseph, the husband of Mary.
The role of prophet is not passed on through blood lines; rather, God calls the prophet. This call and public commission by God the Father was given to Jesus after his baptism by John, when the Holy Spirit descended upon him and drove him into the desert to be tempted.
But what about the priestly office? How does Jesus lay claim to the priesthood in human terms? There were two kinds of priesthood under the Torah, and both were handed on by bloodline. Intercession between God and man was entrusted to the descendants of Aaron, and they were assisted by the tribe of Levi.
If Mary, like her cousin Elizabeth, was descended from Aaron, then it all gets wrapped together nicely. The royal claim is through Joseph, the priestly claim is through Mary, and the prophetic claim is through the events of the Baptism.
The Catholic Encyclopedia entry for the Blessed Virgin Mary indicates that Mary might have been both. Her father, Joachim, might have been a descendent of David and her mother, Anne, a descendent of Aaron. For the most part, though, the most we can do regarding the lineage of Mary is speculate.