9 We believe that Mary, as well as Joseph, was of the family of David, because we believe the Scriptures, which assert both that Christ was of the seed of David after the flesh, and that His mother was the Virgin Mary, He having no human father. Therefore, whoever denies the relationship of Mary to David, evidently opposes the pre-eminent authority of these passages of Scripture; and to maintain this opposition he must bring evidence in support of his statement from writings acknowledged by the Church as canonical and catholic, not from any writings he pleases.
In the matters of which we are now treating, only the canonical writings have any weight with us; for they only are received and acknowledged by the Church spread over all the world, which is itself a fulfillment of the prophecies regarding it contained in these writings. Accordingly, I am not bound to admit the uncanonical account of Mary's birth which Faustus adopts, that her father was a priest of the tribe of Levi, of the name of Joachim. But even were I to admit this account, I should still contend that Joachim must have in some way belonged to the family of David, and had somehow been adopted from the tribe of Judah into that of Levi; or if not he, one of his ancestors; or, at least, that while born in the tribe of Levi, he had still some relation to the line of David; as Faustus himself acknowledges that Mary, though belonging to the tribe of Levi, could be given to a husband of the tribe of Judah; and he expressly says that if Mary were Joseph's daughter, the name Son of David would be applicable to Christ.
In this way, by the marriage of Joseph's daughter in the tribe of Levi, her son, though born in the tribe of Levi, might not improperly be called the Son of David. And so, if the mother of that Joachim, who in the passage quoted by Faustus is called the father of Mary, married in the tribe of Levi while she belonged to the tribe of Judah and to the family of David, there would thus be a sufficient reason for speaking of Joachim and Mary and Mary's son as belonging to the seed of David.
If I felt obliged to pay any regard to the apocryphal scripture in which Joachim is called the father of Mary, I should adopt some such explanation as the above, rather than admit any falsehood in the Gospel, where it is written both that Jesus Christ, the Son of God, and our Saviour, was of the seed of David after the flesh, and that He was born of the Virgin Mary. It is enough for us that the enemies of these Scriptures, which record these truths and which we believe, cannot prove against them any charge of falsehood.
Source: Reply to Faustus the Manichaean (New Advent)