“Bring forth therefore fruit worthy of repentance, and think not to say within yourselves, ‘We have Abraham to our father,' for I say unto you, that God is able of these stones to raise up children to Abraham.”
These words are probably now more generalised. All who are listening to him are therefore to bring forth fruit which is worthy of ‘repentance', of indicating that their hearts and minds are truly changed (truly repentant) by bringing forth fruit which will indicate that God has rained on them with the water of His word and Spirit (Isaiah 32:15; Isaiah 44:1-5; Isaiah 55:10-11; Ephesians 5:26; Titus 3:5), as his baptism indicates. Serpents were always looked on as worldly wise (Matthew 10:16). That might mean that he saw the purpose of some of these who came to him for baptism as a rather naive way of attempting to obtain blessing without true response.
Nor were they to assume that because they could claim Abraham as their father all would be right. They needed to recognise that being ‘a son of Abraham' was of no value unless they believed and walked like Abraham. Indeed let them recognise that God could even take the stones that they saw around them, and could turn them into sons of Abraham.
Many Israelites did in fact believe that being a pure-bred son of Abraham would mean that their inheritance in the eternal Kingdom was ensured. And they regularly ensured marriage with similarly minded people in order to preserve their position. John is making quite clear that this was not so. (As a priest's son he could not be accused of sour grapes, for it meant that his own lineage would be seen as pure). Their hearts had to be genuine, for let them not be in any doubt, God was not restricted in whom He could turn into sons of Abraham. While John probably mainly had in mind the tax-gatherers and sinners, and those of despised trades, the fact that he also welcomed soldiers suggest that he was not averse to including some Gentiles, for local auxiliary soldiers would be mainly local Gentiles.
The idea of a connection with stones may spring from Isaiah 51:1 where Israel were told to seek the Lord and look to the rock from which they were hewn and the quarry from which they were dug, namely to Abraham their father. Thus Abraham was there seen as a rock from which stones were hewn. This could then be a sarcastic statement that they should recognise the folly of their position. God can produce children to Abraham from any kind of rocks. Coming from Abraham means no more than coming from the rocks around them, unless their hearts are like Abraham's. Thus being a son of Abraham counts for nothing unless they walk in his ways (compare Galatians 3:6-9; Galatians 3:29; Romans 4).
He may also have been influenced by the similarity between abnayya (stones) and benayya (children) in Aramaic thus saying sarcastically ‘from these abnayya God can raise up benayya' (John would be speaking in Aramaic), just as He had previously raised them up from the rock that bore them. And those raised up from the stones would then have the same standing before God, for it was not physical birth from Abraham that counted, it was spiritual birth. It was in a sense prophetic. For God would in future raise up sons to Abraham from among the Gentiles who became his sons through faith (Galatians 3:29).
So he makes clear that his baptism will be totally ineffective unless their lives and hearts are changed. Those who would come for baptism must have begun (or have determined to begin) fruit-bearing lives or their baptism will mean nothing.