Albert Barnes Commentary Matthew 21:27

"And they answered Jesus, and said, We know not. He also said unto them, Neither tell I you by what authority I do these things." — Matthew 21:27 (ASV)

We cannot tell. This was a direct falsehood. They could have told; and it should have been, we will not tell. There was no reason why they did not tell, except that they were unwilling. The reason probably why they would not acknowledge that John was a prophet was that, if they did, they saw he could easily show them by what authority he did those things; i.e., as Messiah. John predicted him, pointed him out, baptized him, came as his forerunner, to fulfill the prophecies. If they acknowledged one, they must the other. In this way our Savior was about to lead these crafty men to answer their own question, to their own confusion, about his authority. They saw this; and having given them a sufficient answer, there was no need of stating anything further.