God is therefore called in particular “The God of Abraham, the God of Isaac, and the God of Jacob,” because in these three are expressed all the modes of begetting the sons of God. For God begets most times of a good preacher a good son, and of a bad preacher a bad son. This is signified in Abraham, who of a free woman had a believing son, and of a bondslave an unbelieving son. Sometimes indeed of a good preacher He begets both good and bad sons, which is signified in Isaac, who of the same free woman begot one good and the other bad. And sometimes He begets good sons both of good and bad preachers; which is signified in Jacob, who begot good sons both of free women and of bondmaids.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: in Joan. Tr., xi, 8