Well, in the preceding verses, they were questioning Jesus about his authority. Jesus answered by asking them about John the Baptist's authority, to draw an analogy. So the obvious answer would be that they didn't believe John's testimony of Jesus:
19 And this is the record of John, when the Jews sent priests and
Levites from Jerusalem to ask him, Who art thou?
20 And he confessed, and denied not; but confessed, I am not the
21 And they asked him, What then? Art thou Elias? And he saith, I am
not. Art thou that prophet? And he answered, No.
22 Then said they unto him, Who art thou? that we may give an answer
to them that sent us. What sayest thou of thyself?
23 He said, I am the voice of one crying in the wilderness, Make
straight the way of the Lord, as said the prophet Esaias.
24 And they which were sent were of the Pharisees.
25 And they asked him, and said unto him, Why baptizest thou then, if
thou be not that Christ, nor Elias, neither that prophet?
26 John answered them, saying, I baptize with water: but there
standeth one among you, whom ye know not;
27 He it is, who coming after me is preferred before me, whose shoe's
latchet I am not worthy to unloose.
28 These things were done in Bethabara beyond Jordan, where John was
29 The next day John seeth Jesus coming unto him, and saith, Behold
the Lamb of God, which taketh away the sin of the world.
30 This is he of whom I said, After me cometh a man which is preferred
before me: for he was before me.
31 And I knew him not: but that he should be made manifest to Israel,
therefore am I come baptizing with water.
32 And John bare record, saying, I saw the Spirit descending from
heaven like a dove, and it abode upon him.
33 And I knew him not: but he that sent me to baptize with water, the
same said unto me, Upon whom thou shalt see the Spirit descending, and
remaining on him, the same is he which baptizeth with the Holy Ghost.
34 And I saw, and bare record that this is the Son of God.