5 But we, dearly beloved, do we come of Abraham's race, or was Abraham in any sense our father according to the flesh? The flesh of the Jews draws its origin from his flesh, not so the flesh of Christians. We have come of other nations, and yet, by imitating him, we have become the children of Abraham. Listen to the apostle: “To Abraham and to his seed were the promises made. He says not,” he adds, “And to seeds, as of many; but as of one, And to your seed, which is Christ.
And if you be Christ's, then are you Abraham's seed, and heirs according to the promise.” We then have become Abraham's seed by the grace of God. It was not of Abraham's flesh that God made any co-heirs with him. He disinherited the former, He adopted the latter; and from that olive tree whose root is in the patriarchs, He cut off the proud natural branches, and engrafted the lowly wild olive. And so, when the Jews came to John to be baptized, he broke out upon them, and addressed them, “O generation of vipers.”
Very greatly indeed did they boast of the loftiness of their origin, but he called them a generation of vipers—not even of human beings, but of vipers. He saw the form of men, but detected the poison. Yet they had come to be changed, because at all events to be baptized; and he said to them, “O generation of vipers, who has warned you to flee from the wrath to come? Bring forth therefore fruits meet for repentance. And think not to say within yourselves, We have Abraham to our father; for God is able of these stones to raise up children unto Abraham.” If you bring not forth fruits meet for repentance, flatter not yourselves about such a lineage.
God is able to condemn you, without defrauding Abraham of children. For He has a way to raise up children to Abraham. Those who imitate his faith shall be made his children. “God is able of these stones to raise up children unto Abraham.” Such are we. In our parents we were stones, when we worshipped stones for our god. Of such stones God has created a family to Abraham.
Source: Tractates on the Gospel of John (New Advent)