John's baptism—was it from heaven, or of human origin? Tell me!”
It seems here that Jesus is inviting to question the John's baptism, in reply to the usual attempt by scribes and pharises to question his own (of Jesus) authority.
The Pharisees's basic reluctance to question the John's baptism is evidence that they oppose John to Jesus. And a such opposition is accepted by Jesus and works subtly as a clue to know who is really Jesus.