Jamieson, Fausset and Brown on Matthew 21:25

25. The baptism of John--meaning his whole mission and ministry, of which baptism was the proper character.

whence was it? from heaven, or of men?--What wisdom there was in this way of meeting their question will best appear by their reply.

If we shall say, From heaven; he will say unto us, Why did ye not then believe him?--"Why did ye not believe the testimony which he bore to Me, as the promised and expected Messiah?" for that was the burden of John's whole testimony.