Matthew 3:7

The doctrine John preached was that of repentance, in consideration of the kingdom of heaven being at hand; now here we have the use of that doctrine. Application is the life of preaching, so it was of John's preaching. Observe, 1. To whom he applied it; to the Pharisees and Sadducees that came to his baptism, Mat 3:7. To others he thought it enough to say, Repent, for the kingdom of heaven is at hand; but when he saw these Pharisees and Sadducees come about him, he found it necessary to explain himself, and deal more closely. These were two of the three noted sects among the Jews at that time, the third was that of the Essenes, whom we never read of in the gospels, for they affected retirement, and declined busying themselves in public affairs. The Pharisees were zealots for the ceremonies, for the power of the church, and the traditions of the elders; the Sadducees ran into the other extreme, and were little better than deists, denying the existence of spirits and a future state.

— Matthew Henry's Concise Commentary (public domain)

But when he saw many of the Pharisees and Sadducees come to his baptism, he said unto them--astonished at such a spectacle. O generation of vipers--"Viper brood," expressing the deadly influence of both sects alike upon the community. Mutually and entirely antagonistic as were their religious principles and spirit, the stem prophet charges both alike with being the poisoners of the nation's religious principles. In Mat 12:34; Mat 23:33, this strong language of the Baptist is anew applied by the faithful and true Witness to the Pharisees specifically--the only party that had zeal enough actively to diffuse this poison. who hath warned you--given you the hint, as the idea is. to flee from the wrath to come?--"What can have brought you hither?" John more than suspected it was not so much their own spiritual anxieties as the popularity of his movement that had drawn them thither. What an expression is this, "The wrath to come!" God's "wrath," in Scripture, is His righteous displeasure against sin, and consequently against all in whose skirts sin is found, arising out of the essential and eternal opposition of His nature to all moral evil.

— Jamieson-Fausset-Brown Commentary (public domain)