Christ taught you can tell what a person is like inside by the words they speak, and that everyone will give account for their words.
This insult appears in the gospels of both Mathew and Luke (Matt 3:7; 12:34; 23:33; Luke 3:7). Both John the Baptist and Jesus used it to denounce the hypocrisy of those who claimed to be righteous and good but who were, in their heart of hearts, unwilling to treat their neighbours with justice and mercy.
John had spent a lot of time in the desert, so he knew very well what dangers lay hidden there. He knew that the varied species of lethal vipers hid under the dry bushes that abound in those solitary landscapes.
He would presumably often see how that dry vegetation would spontaneously burst into flame due to the heat and the sparks caused by human activity. Then the camouflaged vipers would come out of their hiding places to escape the fire.
This is precisely the image that John used to refer to many of those who came to be baptised, “You brood of vipers! Who taught you to flee the coming wrath?”
The Jews believed they were superior to other peoples, and they were convinced that when God judged them, he would be more benevolent towards them than everyone else.
However, John told them that they were just like those dangerous snakes that come out of their hiding places to get baptised for fear of divine judgement but, in reality, they were not willing to change their lives.
They thought that baptism was a kind of magic ritual that would automatically cleanse them of their sins. However, the toxin of their sin remained deep in their hearts because they did not sincerely repent of it.
Hence the harshness of the Baptist’s rebuke. The Lord Jesus also called the scribes and the Pharisees a brood of vipers because they had accused the Master of casting out demons by Beelzebub, the prince of demons (Matt. 12:24).
To say that the Son of God was in league with the devil was a terrible insult, as it would poison the minds of the hearers and sow doubt in their hearts.
That is why Christ taught you can tell what a person is like inside by the words they speak, and that everyone will give account for their words. “For from the abundance of the heart the mouth speaks”.
The mouth can exhale words that build up, but also words that poison and destroy. Did that ancient brood of vipers become extinct, or are some of its descendants still alive and well today?