Luke 20:8

And Jesus said unto them, Neither tell I you by what authority I do these things.

Jesus declined to tell the rulers and priests by what authority He acted. The challenge concerned His right to do the works He was doing. His authority was already evident in His divine mission and in His command to cleanse the temple, when no one had dared to question Him as He drove the traders out.

He made His own answer conditional on their answering His counter-question, whether the baptism of John was from heaven or from men. They evaded that question and refused to answer it. Their refusal exposed their unwillingness to acknowledge John's divine commission and his testimony that Jesus was the Messiah, since to admit that John's mission was divine would have required them to face that testimony.

By that evasive reply they forfeited any right to demand His answer. His reply, withholding His authority in return, exposed their evasion. It left them confused and diminished in the eyes of the people.