Luke 20:5

And they reasoned with themselves, saying, If we shall say, From heaven; he will say, Why then believed ye him not?

The leaders reasoned privately among themselves that if they answered that John's authority was from heaven, they would be met with the question why they had not believed him. They recognized the trap in the answer, for to admit a heavenly origin would expose their failure to believe John's testimony. John had repeatedly pointed to Jesus as the Christ, so to acknowledge John as a prophet would have required them to receive Jesus as the Messiah. John had testified that Jesus is the Lamb of God who takes away the sin of the world, so to accept John's witness would have bound them to acknowledge Jesus as the Messiah.

Their reply was governed not by honest conviction but by fear of being forced to account for their unbelief. Even to these unbelieving men evidence of divine mission was still presented. That witness was given although effort to reform the corrupt priesthood was known to be in vain.