Matt. III. 7.
“But when he saw many of the Pharisees and Sadducees come to his baptism, he said unto them, O generation of vipers, who has warned you to flee from the wrath to come?”
How then does Christ say, that they did not believe John? Because this was not believing, to decline receiving Him whom he preached. For so they thought they regarded their prophets and their lawgiver, nevertheless He said they had not regarded them, forasmuch as they received not Him, that was foretold by them. “For if you had believed Moses,” says He, “ye would have believed Me.” And after this again, being asked by Christ, “The baptism of John, whence is it?” they said, “If we shall say, Of earth, we fear the people; if we shall say, From heaven, He will say unto us, How then did ye not believe him?”
So that from all these things it is manifest that they came indeed and were baptized, yet they did not abide in the belief of that which was preached. For John also points out their wickedness, by their sending unto the Baptist, and saying, “Are you Elias? Are you Christ?” wherefore he also added, “they which were sent were of the Pharisees.”
“What then? Were not the multitudes also of this same mind”? One may say. Nay, the multitudes in simplicity of mind had this suspicion, but the Pharisees, wishing to lay hold of Him. For since it was acknowledged that Christ comes out of the village of David, and this man was of the tribe of Levi, they laid a snare by the question, in order that if he should say any such thing they might quickly come upon him. This at any rate he has declared by what follows; for on his not acknowledging any of the things which they expected, even so they take hold of him, saying, “Why do you baptize then, if you be not the Christ?”
And to convince you that the Pharisees came with one mind, and the people with another, hear how the evangelist has declared this too; saying of the people, “that they came and were baptized of him, confessing their sins;” but concerning the Pharisees, no longer like that, but that “when he saw many of the Pharisees and Sadducees coming, he said, O generation of vipers, who has warned you to flee from the wrath to come?” O greatness of mind! How does he discourse unto men ever thirsting after the blood of the prophets, and in disposition no better than serpents! How does he disparage both themselves and their progenitors with all plainness!
Source: Homilies on the Gospel of St. Matthew (New Advent)