Matthew 12:38-39.
“Then certain of the Scribes and Pharisees answered Him, saying, Master, we would see a sign from You. But He answered and said, An evil and adulterous generation seeks after a sign, and there shall no sign be given to it, but the sign of the Prophet Jonas.”
Could then anything be more foolish than these men (not more impious only), who after so many miracles, as though none had been wrought, say, “We would see a sign from You?” With what intent then did they so speak? That they might lay hold of Him again. For since by His words He had stopped their mouths, once and twice and often, and had checked their shameless tongue, they come to His works again. At which also the evangelist marvelling again, said,
“Then certain of the scribes answered Him, asking a sign.”
“Then,” when? When they ought to be stooping before Him, to admire, to be amazed and give way, “then” they desist not from their wickedness.
And see their words too, teeming with flattery and dissimulation. For they thought to draw Him towards them in that way. And now they insult, now they flatter Him; now calling Him a demoniac, now again “Master,” both out of an evil mind, how contrary soever the words they speak.
Wherefore also He rebukes them severely. And when they were questioning Him roughly and insulting Him, He reasoned with them gently; when they were flattering; reproachfully, and with great severity; implying that He is superior to either passion, and is neither at the one time moved to anger, nor at the other softened by flattery. And see His reproach, that it is not merely hard words, but contains a demonstration of their wickedness. For what says He?
“An evil and adulterous generation seeks after a sign.” Now what He says is to this effect: What marvel if you behave so to me who have been hitherto unknown to you when even to the Father, of whom you have had so much experience, you have done the very same? Forsaking Him, you have run unto the devils, drawing to yourselves wicked lovers. With this Ezekiel too was continually upbraiding them.
Now by these sayings He signified Himself to be of one accord with His Father, and them to be doing nothing new; He was also unfolding their secrets, how with hypocrisy and as enemies they were making their demand. Therefore He called them “an evil generation,” because they have been always ungrateful towards their benefactors; because upon favors they become worse, which belongs to extreme wickedness.
And He called it “adulterous,” declaring both their former and their present unbelief; whereby He implies Himself again to be equal to the Father, if at least the not believing Him makes it “adulterous.”
Source: Homilies on the Gospel of St. Matthew (New Advent)