8 – οὖν – John, having identified the Pharisees and Sadducees as wicked, does not simply tell them to leave, or refuse to baptize them, but urges them to bring their works in line with their baptisms.
9 – “out of these stones God is able to raise up children for Abraham” – this is an allusion to Isaiah 51:1-3, which says, “Look to the rock from which you were hewn, and to the hole of the pit from which you were dug. Look to Abraham your father and to Sarah who bore you…” This is a passage, moreover, that is addressed to “those who seek The Lord”, which is what those coming to John’s baptism are doing. It also proceeds to say that YHWH “will comfort all her waste places; He will make her wilderness like Eden, and her desert like the garden of the Lord.” John is “in the desert of Judea”. Those who came to his baptism were expecting the end of Israel’s exile; they were hoping for precisely the consolation promised in passages like Isaiah 51. That is to say, they have Isaiah 51 running through their minds as they come to John.
For him to tell his hearers not to say “we have Abraham for our father” is thus a direct slap in the face to Jews waiting for the consolation of Israel. Isaiah says, “Look to Abraham your father”, but John says, “Do not imagine you can say, ‘We have Abraham for our Father'”; Isaiah says, “You were quarried out of the rock of Abraham, so you’re a chip off the old block”, but John says, “God can get new children for Abraham out of any rock He pleases. He doesn’t need you.”
The lesson is consistent with Scripture’s warnings against formalism. Circumcision was instituted by God to be a real benefit to each Israelite, a covenantal promise that he belonged to YHWH. But for an unbelieving Jew, all it does is mark him as a transgressor of the Torah (Romans 2:27). The Temple was instituted by God to be a place where He could dwell with Israel, where the children of Abraham could seek His face, but when it is polluted by Israel’s unfaithfulness, it becomes a reason for God to remove them from His presence.
So it is with baptism, the Lord’s Supper, the Bible, prayer, and the worship of the Church: there can be no formalism, no presumption. We are in a real relationship with a real Person. Formalism is not a real relationship. A woman who cheats on her husband has no right to tell him, “But you have to love and respect me because you gave me this wedding ring to wear on my finger.” Yet it would be a mistake to think that, because a wedding ring cannot compel love, therefore it also has no effect on the relation between a husband a wife. It is a symbol, to be sure, but that is not a reason to think it is powerless. Its power is the power that symbols have. If you want to move a heap of dirt or cut wood, symbols won’t get you very far. But in the realm of relationships, they are very potent indeed.