The baptism of John ( το Ì βα ìπτισμα το Ì Ἰωα ìννου). By "the baptism which was of John" Christ means his whole ministry, doctrine, preaching, etc.; as by circumcision is implied the whole Mosaic Law, and the doctrine of the cross comprises all the teaching of the gospel, the chief characteristic connoting all particulars.
From heaven, or of men? Did they regard John as one inspired and commissioned by God, or as a fanatic and impostor, who was self-sent and had received no external authorization? Now, two facts were plain and could not be denied.
The rulers and the people with them had allowed John to be a prophet, and had never questioned his claims hitherto. This was one fact; the other was that John had borne unmistakable evidence to Christ.
"Behold the Lamb of God!" etc. (John 1:32-36), he had said. He came and asserted that he came as Christ's forerunner; his mission was to prepare Christ's way, and had no meaning or intention but this.
Here was a dilemma. They had asked for Jesus' credentials; the prophet, whose mission they had virtually endorsed testified that Jesus was the Messiah; if they believed that John spoke by inspiration, they must accept Christ; if now they discredited John, they would stultify themselves and endanger their influence with the people.
They reasoned with themselves ( παρ ἑαυτοῖς). The somewhat unusual introduction of this preposition instead of the more common ἐν implies that the reflection was not confined to their own breast, but passed in consultation from one to another.
They saw the difficulty, and deliberated how they could meet it without compromising themselves, seeking, not truth, but evasion. Why did ye not then ( διατι ì οὖν: why then did ye not) believe him?
i.e. when he bore such plain testimony to me.