Who John B is

John 1:19-28 ... again
The author wants us to be clear here: John the Baptiser was important, but he was definitely not the person we were introduced to in the prologue.  
All very well, but in that case, who is he, and why is he in our story at all?

Fortunately the Jewish leaders who had been sent to ask him who he was didn't like a non-answer any more than we do.  In fact nobody is keen on uninformative replies.  So, they ask, and we get to hear John B's self-definition.

"I am the voice ..."
He uses a passage of the Bible (what we call the Old Testament, which was all there was at the time, since what we call the New Testament is all about Jesus) and continues his stand for humility.  He is just a voice; the only important thing is the message, the words that point to someone else.

Of course this does not satisfy the questioners.  John B is acting outside their paradigm; he is using a symbol (baptism) in a way that doesn't fit with their expectations.  They want to know how he dares to do this.  His answer is simple: Just wait till you see who's coming!

Right from the first of the events he records for us, our author is helping us to connect with the idea that Jesus is above and beyond what people were expecting.  They were waiting for God to send someone who would baptise.  So here comes a person doing just that ... but what they are hoping for is going to turn out to be more than they have bargained for.
"I baptise with water ..." yes, well that is what baptism is ... ceremonial washing.  Water.
'... but someone is coming after me who is much greater than I am.'  Someone is already here, but unrecognized by you.  I shouldn't even be allowed to serve him, he is so great.

This is an interesting slight on John's part.  These are important men in the social and religious life of the nation, but John refuses to give them a straight answer; more, what he does say implies that they have no understanding of what God is doing.

In telling us about this interaction, the writer does several things at once, and this is his usual style - multiple layers in every choice.  Most obviously, we are given information about Jesus and John - things we already know from the prologue, but placed here in their local context.  John is preparing the way for Jesus so that it will be easier for people to accept him.  Jesus is super-important in comparison to John.

But, we are also introduced to some important themes that will follow us as we read.  The leaders of the people are the very one who will misunderstand Jesus, who will be unable to let go of their preconceived ideas.  More than that, they are potentially hostile to God doing things outside their system; they want things nicely boxed.

It is rather intriguing to notice, before we leave off this part of the story, that John seems to purposefully refrain from telling these men anything useful.  " ... among you stands one you do not know."  It is only the next day that he identifies Jesus, and gives people any real information about him.  Any of yesterday's messengers who were genuinely interested might have stayed to hear, but the message to the leaders was enigmatic in the extreme!

It's like that for us readers too ... we have to come back for more, if we want it.