Who was John the Baptist - John 1:19-34

In this section of the gospel we learn a few things, but we really have a lot of reading to appreciate it completely.

1. John the Baptist was a really big deal.  He was preaching a hard message, but the people loved him.  He was hard against corruption, and he preached repentance beginning with baptism as an act of ritual cleansing.  He was popular enough that the pharisees thought he might have been the messiah and they asked him. John denied it, so they asked if he was Elijah.  He denied that as well.  Jesus disagreed with him on this point.  In Matthew 11 Jesus clearly identifies John the Baptist as Elijah.  We can resolve this dissonance in a couple of ways.  It is possible that John knew that the pharisees were asking if he was physically and actually Elijah, which he was not, so he denied it.  It is also possible that John simply did not know that he was to play that role.  In either case John does identify as the voice in the wilderness from Isaiah 40.
2. Isaiah 40 is a deep rabbit hole of goodness.  I went back to read up on Isaiah 40 thinking I would quickly teach on that as well, only to find that it would require its very own series.  I will offer some context and then invite you to read it in tandem with the Gospel of John.  Just before chapter 40 Isaiah just informed the King Hezekiah that his actions of showing off the kingdom to ambassadors from Babylon would result in Babylon taking over and destroying his kingdom.  The text then moves into a long poem or song that continues for the rest of the book of Isaiah.  There are a few brief breaks in thought, but it keeps cycling over the same topics.  Further reading reveals that this section of Isaiah is like a New Testament Game Plan.  You can't just read a little...you gotta go all the way.  Its good and the gospel revisits these topics over and over.  Take a short break from John and go read Isaiah 40 to the end of the book for context before continuing the New Testament study.
3. Lamb of God is not a penal substitution concept.  When John sees Jesus he introduces him as the Lamb of God who will take away the sin of the world.  We automatically hear that as penal substitution imagery, but it would not have been heard that way, and it is unlikely John meant it that way.  At the very least he would have called him the Bull of God, because the atonement from a bull was more complete and lasted longer.  The symbolism would also require that the offering be burned, not eaten, as is the symbolism of Christ.  The Old Testament has two major references for "lamb of God" that John's statements would have brought to mind.
1. Isaiah 53.  This is a haunting scripture.  It was written long before Christ and even before Crucifixion was well known, but it clearly spells out the Crucifixion of Jesus.  What is lacking is a focus on substitution theory in the text.  The text highlights the way in which the servant of God dies.  Jesus died willingly and he peacefully went to the cross.  It does talk about him taking on sin, but the emphasis is added by translators.  I believe that you could say he suffered as the result of our sin.  Because of the fallen world we live in he did not come in victory, but first had to come in suffering.  This was a new idea for the Jewish people, and Isaiah was introducing it to them. John was reminding them that the first appearance of the messiah was as a suffering servant, not as a victorious king.
2. Genesis 22:8.  This is the attempt of Abraham to sacrifice his son.  I believe that Abraham had gone astray, he thought God told him to sacrifice his son, but God would not ask for that.  The other religions  of that time commonly sacrificed their children for various reasons.  It was not a big leap for Abraham to believe he needed to do the same.  God provided a replacement lamb so that Abraham's zealousy did not lead to the death of his son.  It is important to note that this was not a sin offering...breaking up the substitution theory more.  John was reminding them that Jesus would save us from our religious zealousy, but stepping in the way and bearing the brunt of their attack.
4. John knew that Jesus was pre-existent. John knew Jesus, they were cousins.  He said that he would not have known that Jesus was the messiah except for the spirit descending on him and staying as he had been told to look for that sign particularly.  He also said that though Jesus came after him that he surpassed him because he was before him.  Can you imagine hearing from God that your cousin was God?  I doubt God could have identified Jesus' deity directly, because of the doubt he would have over the message.  John needed to develop his Christology before the identifying who the Christ was so that his own self could be separated from the revelation.  These versus indicate that John knew some things about the Messiah that are not often appreciated on this side of history.  Most Jewish folks believed in a human king messiah.  John not only knew that he would suffer, but knew he was something more, something from before.  He already identified that the Targum's "The Word" was in fact the same as the suffering "Servant of God", and "The Christ".  He was in on the secret that it was in fact God that would fellowship with his creation, even if he had to first empty himself and be rejected.