11 Accordingly we have now found, brethren, of the good, good— of the free woman, Isaac; and of the evil, evil— of the bond woman, Ishmael; and of the good, evil— of Rebecca, Esau: where shall we find of the evil, good? There remains Jacob, that the completion of these four sorts may be concluded in the three patriarchs. Jacob had for wives free women, he had also bond women: the free bear children, as do also the bond, and thus come the twelve sons of Israel. If you count them all, of whom they were born, they were not all of the free women, nor all of the bond women; but yet they were all of one seed.
What, then, my brethren? Did not they who were born of the bond women possess the land of promise together with their brethren? We have there found good sons of Jacob born of bond women, and good sons of Jacob born of free women. Their birth of the wombs of bond women was nothing against them, when they knew their seed in the father, and consequently they held the kingdom with their brethren. Therefore, as in the case of Jacob's sons, that they were born of bond women did not hinder their holding the kingdom, and receiving the land of promise on an equality with their brothers; their birth of bond women did not hinder them, but the father's seed prevailed: so, whoever are baptized by evil men, appear as if born of bond women; nevertheless, because they are of the seed of the Word of God, which is figured in Jacob, let them not be cast down, they shall possess the inheritance with their brethren.
Therefore, let him who is born of the good seed be without fear; only let him not imitate the bond woman, if he is born of a bond woman. Do not imitate the evil, proud, bond woman. For how came the sons of Jacob, that were born of bond women, to possess the land of promise with their brethren, while Ishmael, born of a bond woman, was cast out from the inheritance? How, but because he was proud, they were humble? He proudly reared his neck, and wished to seduce his brother while he was playing with him.
Source: Tractates on the Gospel of John (New Advent)