16 To the sixth age belongs John the Baptist, than whom none greater has arisen among those born of women; of whom it was said, that he was “greater than a prophet.” And how did John show that Christ was sent to all nations? When the Jews came to him to be baptized, that they might not pride themselves on the name of Abraham, he said to them, “O generation of vipers, who has proclaimed to you to flee from the wrath to come? Bring forth therefore fruit worthy of repentance;” that is, be humble; for he was speaking to proud people.
But whereof were they proud? Of their descent according to the flesh, not of the fruit of imitating their father Abraham. What said he to them? “Say not, We have Abraham for our father: for God is able of these stones to raise up children to Abraham.” Meaning by stones all nations, not on account of their durable strength, as in the case of that stone which the builders rejected, but on account of their stupidity and their foolish insensibility, because they had become like the things which they were accustomed to worship: for they worshipped senseless images, themselves equally senseless.
“They that make them are like them, and so are all they that trust in them.” Accordingly, when men begin to worship God, what do they hear said to them? “That ye may be the children of your Father who is in heaven; who makes His sun to rise on the good and on the evil, and sends rain on the just and on the unjust.” Wherefore, if a man becomes like that which he worships, what is meant by “God is able of these stones to raise up children unto Abraham”? Let us ask ourselves and we shall see that it is a fact.
For of those nations are we come, but we should not have come of them had not God of the stones raised up children unto Abraham. We are made children of Abraham by imitating his faith, not by being born of his flesh. For just as they by their degeneracy have been disinherited, so have we by imitating been adopted. Therefore, brethren, this prophecy also of the sixth water-pot extended to all nations; and hence it was said concerning all, “containing two or three metretæ apiece.”
Source: Tractates on the Gospel of John (New Advent)