Jesus saith unto them, Verily I say unto you, That the publicans and the harlots go into the kingdom of God before you. For John came unto you in the way of righteousness, and ye believed him not: but the publicans and the harlots believed him: and ye, when ye had seen it, repented not afterward, that ye might believe him. [Matthew 21 :31,32 KJV]
There was nothing difficult at all about Jesus' question. Either John was, or was not, sent of God, as John tells us he definitely was :
There was a man sent of God whose name was John [John 1:6 KJV]
Was the baptism with which John baptised of heaven ? Then receive it, welcome it, and believe in him to whom John pointed :
Make straight the way of the Lord [John 1:23 KJV]
Behold the Lamb of God which taketh away the sin of the world [John 1:29 KJV]
And I saw and bare record that this is the Son of God [John 1:34 KJV]
But they did not repent. And they did not believe.
Why not ? Because it was not of heaven ?
But they would not be drawn on whether it was, or was not, of heaven.
We cannot tell.
But this was not ingenuous. Later in the chapter, Matthew records the real reason for their not following John, and then Jesus :
Come, this is heir, let us kill him [Matthew 21:38 KJV]
They wanted the things of God for themselves. They wanted to retain their religious hierarchy, their religious position, their self-righteousness. This is what Matthew's book is all about : the inception of the kingdom of heaven and its rejection by a religious kingdom on earth.
Clinging to what had, originally, been established by God (but only as a pre-empting of something far greater, they rejected John, they rejected Jesus, they followed not the publicans and harlots who repented and believed.
And in the end, they crucified the Lord of Glory. (I Corinthians 2:8 KJV.)
Hard-hearted and unbelieveing to the end, the rejected every approach by God and they shunned every means that he gave for them to obtain eternal life by Jesus Christ.
Jesus' question exposes them for their hypocrisy. Nor does he leave it at that. He pursues after them, with a parable which further lays bare their true spiritual state.