3 “And Judas begot Phares and Zara of Thamar.” “What doest thou, O man, putting us in remembrance of a history that contains an unlawful intercourse?” But why is this said? Since, if we were recounting the race of a mere man, one might naturally have been silent touching these things; but if of God Incarnate, so far from being silent, one ought to make a glory of them, showing forth His tender care, and His power. Yea, it was for this cause He came, not to escape our disgraces, but to bear them away. Therefore as He is the more admired, in that He not only died, but was even crucified (though the thing be opprobrious, yet the more opprobrious the more does it show Him full of love to man), so likewise may we speak touching His birth; it is not only because He took flesh upon Him, and became man, that we justly stand amazed at Him, but because He vouchsafed to have also such kinsfolk, being in no respect ashamed of our evils. And this He was proclaiming from the very beginnings of His birth, that He is ashamed of none of those things that belong to us; while He teaches us also hereby, never to hide our face at our forefathers' wickedness, but to seek after one thing alone, even virtue. For such a man, though he have an alien for his ancestor, though he have a mother who is a prostitute, or what you will, can take no hurt thereby. For if the whoremonger himself, being changed, is nothing disgraced by his former life, much more will the wickedness of his ancestry have no power to bring to shame him that is sprung of an harlot or an adulteress, if he be virtuous.
But he did these things not only to instruct us, but also to bring down the haughtiness of the Jews. For since they, negligent about virtue in their own souls, were parading the name of Abraham, thinking they had for a plea their forefathers' virtue; he shows from the very beginning that it is not in these things men ought to glory, but in their own good deeds.
Besides this, he is establishing another point also, to show that all are under sin, even their forefathers themselves. At least their patriarch and namesake is shown to have committed no small sin, for Thamar stands against him, to accuse his whoredom. And David too had Solomon by the wife whom he corrupted. But if by the great ones the law was not fulfilled, much more by the less. And if it was not fulfilled, all have sinned, and Christ's coming has become necessary.
For this cause he made mention also of the twelve patriarchs, by this again bringing down their pride at the noble birth of their fathers. Because many of these also were born of women that were slaves; but nevertheless the difference of the parents did not make a difference in the children. For all were equally both patriarchs and heads of tribes. For this is the precedence of the Church, this the prerogative of the nobility that is among us, taking its type from the beginning. So that whether thou be bond or free, you have from thence nothing more nor less; but the question is all about one thing only, namely, the mind, and the disposition of the soul.
Source: Homilies on the Gospel of St. Matthew (New Advent)