‘He said therefore to the multitudes who went out to be baptised by him, “You offspring of vipers, who warned you to flee from the wrath to come? Bring forth therefore fruits worthy of repentance, and begin not to say within yourselves, “We have Abraham to our father”, for I say to you, that God is able of these stones to raise up children to Abraham.” '
The psalmists liken men to vipers because of the venom of their mouths (Psalms 58:4; Psalms 140:3) and because of their deafness in the face of entreaty (Psalms 58:4). Thus he is warning his listeners not to be like their fathers. The idea here was also of the snakes who fled from the cornfields when the stubble was burned. So what he is saying is that it is useless for his hearers to be like snakes who merely flee from the flames, but are deaf to entreaty. They are rather to undergo a real change of heart and mind. For the wrath to come is not so easily avoided. They are to bring forth fruit which is worthy of indicating that their hearts and minds are truly changed (truly repentant) which will indicate that God has rained on them, 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 those who came to him for baptism as a clever way of getting blessing without true response.
Furthermore they are not to assume that because they can claim Abraham as their father all will be right. Being a son of Abraham is of no value unless they walk like Abraham. Indeed let them recognise that God could even make the stones that they could see round about them into sons of Abraham. This idea may spring from Isaiah 51:1 where they are told to seek the Lord and look to the rock from which they were hewn and the quarry from which they were dug, namely Abraham their father. Thus Abraham is there seen as a rock. It would then be a sarcastic statement that they should recognise the folly of their position. 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 have the same standing before God. 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.