Main point summary
John's testimony, the reason he baptizes, is that there is someone unknown among the Jews who is far greater than him.
And this is the o testimony of John,
when the Jews sent priests and Levites from Jerusalem to ask him,
p “Who are you?”
q He confessed,
and did not deny,
“I am not the Christ.”
And they asked him, “What then? r Are you Elijah?”
He said, “I am not.”
“Are you s the Prophet?”
And he answered, “No.”
So they said to him,
“Who are you?
We need to give an answer to those who sent us.
What do you say about yourself?”
“I am t the voice of one crying out in the wilderness,
‘Make straight 1 the way of the Lord,’
as the prophet Isaiah said.”
(Now they had been sent from the Pharisees.)
They asked him,
u “Then why are you baptizing,
if you are neither the Christ,
nor the Prophet?”
John answered them,
v “I baptize with water,
but among you stands one you do not know,
even w he who comes after me,
the strap of whose sandal I am not worthy to untie.”
These things took place in Bethany across the Jordan, where John was baptizing.
This links into 22c via idea/explanation, as it's explaining that the pharisees are the ones who sent them. It could also be used as the joint for a bilateral relationship to the two large question blocks above and below it. That is to say, everything the Jews ask here, they ask because the Pharisees sent them. However, the fact that the Pharisees sent them is less important than highlighting the relationship between those two big blocks of questions, so I'm adding it on to the second block.
This is effectively an action/purpose bilateral. "We need to give an answer" is the purpose to asking the questions "Who are you", and "What do you say about yourself?"
The relationship between the series of questions here is interesting. It could be argued as a series, or as a progression leading up to the final one that he answers in full with his identity. However, consider the second half of this passage. They ask him why he baptizes, if he's not the Christ, or Elijah, or the Prophet. And there's also the question of he answered no to all those things, considering Jesus later identifies him as the Elijah who was to come! I think the key point is this. They were looking for a place to slot him into their worldview; for him to literally be Elijah or to conform to their predefined notions of Christ or the Prophet. But that worldview is not correct. He takes a fourth option, and uses a quote from Isaiah to define his role. This also informs the relationship between this and the final block where John explains why he baptizes. Had he claimed to be Christ, or Elijah, or the Prophet, this question would never of occurred! Of course those three would baptize! But because he denies being any of those, they have to ask why. The question of why he baptizes is therefore a direct result of how he answered the question of who he is.
After asking who John is, after a bunch of false starts on whether he's Christ, or Elijah, or the Prophet, they then ask why he baptizes. What's the point of his ministry? And the crux of John's answer? There is someone among them who they do not know. Someone who comes after him. Someone so great, he's unworthy to even untie his sandal! This seems like a very odd way to answer the question of why he baptizes, but this is the key statement that John's testimony leads up to. And the next passage (that is to say, verses 29-34) will expand on that more, showing why that's the reason for John's ministry. But this block of verses here is setting the stage. John is telling the Jews and his disciples that he is not the important one. Someone greater is coming.