7 And now what answer did they give Him? For they began somewhat to realize that the Lord was not speaking of carnal generation, but of their manner of life. And because it is the custom of the Scriptures, which they read, to call it, in a spiritual sense, fornication, when the soul is, as it were, prostituted by subjection to many false gods, they made this reply: “Then said they to Him, We be not born of fornication; we have one Father, even God.” Abraham has now lost his importance. For they were repulsed as they ought to have been by the truth-speaking mouth; because such was Abraham, whose deeds they failed to imitate, and yet gloried in his lineage. And they altered their reply, saying, I believe, with themselves, As often as we name Abraham, he goes on to say to us, Why do ye not imitate him in whose lineage ye glory? Such a man, so holy, just, and guileless, we cannot imitate. Let us call God our Father, and see what he will say to us.
Source: Tractates on the Gospel of John (New Advent)