The leaders were not looking for the true answer. They were looking for a safe one. Before they replied to Jesus, they talked it over among themselves and traced where each answer would lead. If they said "From heaven," they knew Jesus would ask this question: then why did you not believe John? Gill says that "from heaven" was the answer which, "in their own consciences, they believed to be true." Barnes, in his notes on the parallel in Matthew 21, gives the probable reason the question frightened them: John had pointed to Jesus, so "If they acknowledged one, they must the other." The question they put in Jesus' mouth was one their own consciences were already asking.

Jesus never had to ask the question aloud, because the leaders asked it for Him. The other answer was no safer: "all the people will stone us: for they be persuaded that John was a prophet" (Luke 20:6). So "they answered, that they could not tell whence it was" (Luke 20:7). Jesus then declined their demand: "Neither tell I you by what authority I do these things" (Luke 20:8).

John preached "the baptism of repentance for the remission of sins" (Luke 3:3), and he told the crowds that One mightier than he was coming (Luke 3:16). Luke had already recorded the split John's baptism caused: the people and the publicans (tax collectors) "justified God, being baptized with the baptism of John," but the Pharisees and lawyers "rejected the counsel of God against themselves, being not baptized of him" (Luke 7:29-30).

These leaders were already seeking a way "to destroy him," but could not, because "all the people were very attentive to hear him" (Luke 19:47-48). Matthew Henry says they were "governed by reputation and secular interest," which is why both answers felt dangerous to them.

The leaders came to judge Jesus' authority, but His one question put them on trial instead. John had been sent by God to prepare the way for the One coming after him. To admit that John's baptism was from heaven was to admit that John's witness about Jesus was true. Their answer, that they could not tell, did not come from a lack of knowledge. It was a refusal to follow what they knew.

That is why Jesus would not answer their demand (Luke 20:8). Matthew Henry puts it plainly: "Those that bury the knowledge they have are justly denied further knowledge." God gives light generously, but He does not hand more light to people who are hiding from the light they already have. Matthew Henry's Concise Commentary adds that men often claim to examine the evidence "when only seeking excuses for their own unbelief and disobedience."