64 Doubtless, the intention of Faustus' calumnies is to damage this very assertion, that Christ was born of the tribe of Judah. Especially, as in the genealogy given by Matthew we find the name of Zara, whom this woman Tamar bore to Judah. Had Faustus wished to reproach Jacob's family merely, and not Christ's birth, he might have taken the case of Reuben the first-born, who committed the unnatural crime of defiling his father's bed, of which fornication the apostle says, that it was not so much as named among the Gentiles. Jacob also mentions this in his blessing, charging his son with the infamous deed.
Faustus might have brought up this, as Reuben seems to have been guilty of deliberate incest, and there was no harlot's disguise in this case, were it not that Tamar's conduct in desiring nothing but to have children is more odious to Faustus than if she had acted from criminal passion, and did he not wish to discredit the incarnation, by bringing reproach on Christ's progenitors. Faustus unhappily is not aware that the most true and truthful Saviour is a teacher, not only in His words, but also in His birth.
In His fleshly origin there is this lesson for those who should believe in Him from all nations, that the sins of their fathers need be no hindrance to them. Besides, the Bridegroom, who was to call good and bad to His marriage, was pleased to assimilate Himself to His guests, in being born of good and bad. He thus confirms as typical of Himself the symbol of the Passover, in which it was commanded that the lamb to be eaten should be taken from the sheep or from the goats— that is, from the righteous or the wicked. Preserving throughout the indication of divinity and humanity, as man He consented to have both bad and good as His parents, while as God He chose the miraculous birth from a virgin.
Source: Reply to Faustus the Manichaean (New Advent)