20:1-8 Men often pretend to examine the evidences of revelation, and the truth of the gospel, when only seeking excuses for their own unbelief and disobedience. Christ answered these priests and scribes with a plain question about the baptism of John, which the common people could answer. They all knew it was from heaven, nothing in it had an earthly tendency. Those that bury the knowledge they have, are justly denied further knowledge. It was just with Christ to refuse to give account of his authority, to those who knew the baptism of John to be from heaven, yet would not believe in him, nor own their knowledge.See this passage explained in the notes at Matthew 21:23-27. 4. baptism of John—his whole ministry and mission, of which baptism was the seal. See Poole on "Luke 20:3"

The baptism of John, was it from heaven, or of men? This was a new ordinance, and John must have his authority for administering it either from God, or from men; and Christ is desirous to know from which he derived it in their opinion; suggesting, that by the same authority John, his forerunner, came baptizing, he himself came preaching and working miracles; See Gill on Matthew 21:25. The baptism of John, was it from heaven, or of men?