8 The patriarchs, then, are these three, Abraham, Isaac, and Jacob. You know that the sons of Jacob were twelve, and thence the people Israel; for Jacob himself is Israel, and the people Israel in twelve tribes pertaining to the twelve sons of Israel. Abraham, Isaac, and Jacob three fathers, and one people. The fathers three, as it were in the beginning of the people; three fathers in whom the people was figured: and the former people itself the present people. For in the Jewish people was figured the Christian people.
There a figure, here the truth; there a shadow, here the body: as the apostle says, “Now these things happened to them in a figure.” It is the apostle's voice: “They were written,” says he, “for our sakes, upon whom the end of the ages has come.” Let your mind now recur to Abraham, Isaac, and Jacob. In the case of these three, we find that free women bear children, and that bond women bear children: we find there offspring of free women, we find there also offspring of bond women.
The bond woman signifies nothing good: “Cast out the bond woman,” says he, “and her son; for the son of the bond woman shall not be heir with the son of the free.” The apostle recounts this; and he says that in those two sons of Abraham was a figure of the two Testaments, the Old and the New. To the Old Testament belong the lovers of temporal things, the lovers of the world: to the New Testament belong the lovers of eternal life. Hence, that Jerusalem on earth was the shadow of the heavenly Jerusalem, the mother of us all, which is in heaven; and these are the apostle's words. And of that city from which we are absent on our sojourn, you know much, you have now heard much.
But we find a wonderful thing in these births, in these fruits of the womb, in these generations of free and bond women: namely, four sorts of men; in which four sorts is completed the figure of the future Christian people, so that what was said in the case of those three patriarchs is not surprising, “I am the God of Abraham, and the God of Isaac, and the God of Jacob.” For in the case of all Christians, observe, brethren, either good men are born of evil men, or evil men of good; or good men of good, or evil men of evil: more than these four sorts you cannot find.
These things I will again repeat: Give heed, keep them, excite your hearts, be not dull; take in, lest ye be taken, how of all Christians there are four sorts. Either of the good are born good, or of the evil, are born evil; or of the good are born evil, or of the evil good. I think it is plain. Of the good, good; if they who baptize are good, and also they who are baptized rightly believe, and are rightly numbered among the members of Christ. Of the evil, evil; if they who baptize are evil, and they who are baptized approach God with a double heart, and do not observe the morals which they hear urged in the Church, so as not to be chaff, but grain, there.
How many such there are, you know, beloved. Of the evil, good; sometimes an adulterer baptizes, and he that is baptized is justified. Of the good, evil; sometimes they who baptize are holy, they who are baptized do not desire to keep the way of God.
Source: Tractates on the Gospel of John (New Advent)