The book of the genealogy of Jesus Christ, son of David, son of Abraham. –Mt 1:1
These first words of the New Testament are followed by the long list of ancestors which ultimately culminates with the following words: “Jacob [became] the father of Joseph, the husband of Mary. Of her was born Jesus who is called the Christ.” Have you noticed that this extended family tree is not actually shown to be our Savior’s lineage? In fact, both Matthew and Luke give the genealogy not of Jesus, but of Joseph. Since Jesus was not truly Joseph’s son, it is Mary’s genealogy which would reveal to us the Savior’s ancestry – But do we have any idea who Mary’s ancestors were?
We know from Sacred Scripture and from Sacred Tradition that Mary was a descendent of David. St. Paul states, “it is evident that our Lord sprung out of Juda” (Hebrews 7:14). Yet, St. Thomas mentions that some may object, for it seems that the Blessed Virgin Mary was not of the tribe of Juda but was indeed of Aaron’s stock, as was her cousin Elizabeth.(Lk 1:5) St. Thomas responds that, as the kingly and priestly family lineages were most highly regarded, they were frequently joined by marriage. Thus, it is not impossible that St. Elizabeth as well as the Blessed Virgin may have had some part in the tribe of Juda. And so too Christ was of the tribe of Juda, as witnessed by the book of the Apocalypse, where Christ is called “the lion of the tribe of Juda.” (Rev 5:5)
St. Thomas makes this same point in the Summa Theologiae when showing that our Savior is truly a descendent of David (and, therefore, of Juda), attributing the theory to St. Gregory Nazianzen (ST III, q.31, a.2, ad 2). In that place, St. Thomas gives a slightly more explicit explanation of this point, stating that it is possible that St. Elizabeth’s father, who was of the tribe of Aaron, might have married a female relative of the Blessed Virgin, who was of the tribe of Juda, and thus, St. Elizabeth and the Virgin Mother would be cousins.
What perhaps is more likely, St. Thomas says, is that St. Joachim may have been of Juda and St. Anne of Aaron and a relative of St. Elizabeth’s father; in this manner as well, the Blessed Virgin would be the cousin of St. Elizabeth and yet also be of the tribe of Juda.
In any case, St. Thomas is content to allow for any explanation, so long as it is maintained that our Lord was a descendent of the tribe of Juda according to the flesh. Clearly, it would not be sufficient to trace our Lord’s lineage to Juda through St. Joseph, since the Christ was not the son of St. Joseph according to the flesh. However, as it is likely that the Blessed Virgin and St. Joseph were of the same tribe – as they were enrolled in the census together (Mt 1:18); therefore, the fact that St. Joseph was of the tribe of Juda does indicate that the Blessed Virgin too was most likely of the same tribe and, therefore, so too was Christ (ST III, q.31, a.2, ad 1).Thus, we can safely conclude that Mary’s genealogy would be fairly close to St. Joseph’s which is given in Matthew. The final generations were, obviously different; but it is not unlikely that Joseph and Mary were distant relatives, both belonging to the tribe of Juda, being descendents of David. Christ then belongs to David’s lineage both according to the Law (through St. Joseph) and according to the flesh (through Blessed Mary).