So, why are we talking about the light?

It is as if a piece of the story escapes; it intrudes into the poetry of the prologue and turns out to fit perfectly.  In fact, by this device of an 'extraneous' comment, John gives us the first indication that this is not going to be a Grand Myth, like a Greek tragedy played out only on the stage.  Introducing a human with a name and a history demonstrates that in spite of the cosmic and global implications, this is the report of something that really happened in space and time, with living people.

A person came from God, the same God that we read about a few sentences ago, and he was a real person called John.  (Note:  This is not our John, the one wrting the record; it is the John which the other records call John the Baptiser.  John was a very common name as it is today, and of course they didn't use surnames.)  God sent him for a reason which has to do with the coming of the light, but our writer makes sure we realise that John (the Baptiser) is NOT the light.  Now, this would be an important thing to clarify for anyone who had doubts, but as far as I can see by the time John the Writer was composing his record this was hardly necessary. 

However, it serves to tell us something deeper.  John the Baptiser was a special person because her was sent from God, which is fairly unique.  But he is sent only as a witness concerning the one introduced as the light.  He is meant to make it possible for other people to believe.  This inserts the realisation that this light is every bit as important as we can possibly guess, since God himself sends a person especially to introduce the light!

The interesting thing is we have still not been told that 'the light' is human.  All we know is that this One, who is identified with God (as the Word) is also crucial to the entire human race.  In the mean time, John has sneaked in another word that will be repeatedly part of his narative: "believe".

Okay, the 'sideline' with it's suggestive information is over, and we return to the abstract metaphysics:
This light John designates as the true light; it is the real thing, genuine and authentic, the actual opposite of darkness.  More than that, he describes it as universal; all people have some measure of light, and it is this light.  We begin to realise that for John, everything else can be considered darkness. 

Now, right at the end of this section, when we have already been given so much that will reverberate in our minds, only now are we told the key to the whole story:  this light was coming into the world.  And so now we are aware of what the fuss is about.

Just in time to move into a section where the whole metaphor of light is completely ignored!