Mark 11:33
And they answered and said unto Jesus, We cannot tell. And Jesus answering saith unto them, Neither do I tell you by what authority I do these things. And they answered and said unto Jesus, We cannot tell. And Jesus answering saith unto them, Neither do I tell you by what authority I do these things.
The priests, rulers and leaders confronted Jesus to demand by what authority He acted, and when He in turn asked them whether John's baptism was from heaven or from men, they answered that they could not tell, so He refused to tell them by what authority He did these things. He made disclosing His authority dependent on their honest decision about John's divine mission, so by evading and refusing to decide they were left without grounds to demand His answer.
The issue turned on John having identified Jesus as the Messiah, so to acknowledge that John was sent from heaven would require accepting his testimony to Jesus, while to deny his divine mission risked angering the people who held John as a prophet. Their professed ignorance exposed their unwillingness to commit themselves and their fear of the people, and their cowardice and indecision cost them the people's respect.