So following from Sunday, I start the book over. Thanks to everyone for things I had missed or needed to be sure to cover under other headings!
We don't know the exact year. Luke 3:1 says it was the 15th year of the Roman emperor Tiberius, around the year AD29. John "the baptizer" started baptizing people in the river Jordan just a few miles east of Jerusalem. Of course dating things is really complicated in the ancient world, so this is not a slam dunk.
Tiberius was the second Roman emperor (AD14-37), after Augustus (31BC-AD14). You probably remember that Augustus was emperor when Jesus was born. You've probably heard the verse at least at Christmas, "In those days Caesar Augustus issued a decree that a census should be taken" (Luke 2:1). Our best guess is that Jesus was born sometime around 6-4BC because Matthew 2 says Herod (the Great) was still king, and Herod the Great died in 4BC.
That means that Jesus would be about thirty, maybe a little
older, when he showed up at the Jordan River to be baptized by John (Luke 3:23;
cf. John 8:57). It is a little irritating not to know these things for
certain, but that's just the way it is with the evidence we have. People
can get really angry when you mention uncertainty about things they've seen
in every Christmas play since childhood. But the gospels do not always say exactly
the same thing when it comes to minor details, so we just can't know for sure
on some of these questions. We just have to deal with it.
What was John doing out there in the middle of nowhere,
baptizing? Jesus presumably came all the
way from Galilee to see him, to participate.
It’s about a three day journey. Jesus must have agreed with most or all of what John was saying to go through and get
baptized by him.
The gospels tell us that John the Baptist was preaching a “baptism of repentance
for the forgiveness of sins” (e.g., Mark 1:4).
The Jewish historian Josephus, as we would expect, presents John’s
message in more general terms. John was
urging the crowds to become more virtuous. 
It's hard for us to get into the heads of John or the crowds. We've heard so much in church and other places. What would we have thought if we had been in those crowds who came to see John and maybe get baptized?
I think we would have heard something distinctly political both in John's message and symbolic action of baptism. It is no coincidence that a new Herod, Herod Antipas, arrested John and eventually beheaded him. He was no fool.
For example, where was John baptizing? He was baptizing on the east side of the Jordan River, right around the place where Joshua had led Israel to occupy the land. In the light of the rest of John's message, it was all too easy to see that John was preparing Israel for its restoration as a free kingdom.
The goal of repentance was to get the hearts of Israel ready for the return of God's kingdom on earth in Israel. It is again no coincidence that the gospels remember John in the light of Old Testament passages like Isaiah 40:1-3. While New Testament authors do not always read the Old Testament in context, the context of Isaiah fits John's message very well indeed: "a voice of one calling in the wilderness, 'Prepare the way for the Lord, make straight paths for him'" (Mark 1:3, quoting Isaiah 40:3).
The original context of Isaiah 40 was the return of Israel from captivity in Babylon. Babylon had destroyed Jerusalem and taken many Israelites back as slaves. But in 538BC, a new conquering king--Cyrus, king of Persia (cf. Isa. 45:1)--let those Jews who wanted to return go. Isaiah 40 originally was about making a straight line through the desert home to Jerusalem from Babylon. Flatten the hills, lift up the valleys, and straighten out any crooked roads because we're going home! 
So if we were there with John by the Jordan, I think we would have heard similar overtones in what he was doing. In a sense, Israel is still in exile. The Romans are in control. These texts in Isaiah point toward Israel regaining its independence and being restored (cf. Acts 1:6).
John probably also criticized the current leadership of Jerusalem. His message of repentance was a message of hope for those who participated. But it was a message of judgment for everyone else, including the current leaders in Jerusalem and people like Herod Antipas. For them it was a message of impending doom...
 Antiquities 18.5.2.
 Accordingly, in its original context, the Lord Isaiah has in mind is Yahweh, the LORD, rather than a king. However, as we will see, it was perfectly legitimate for the New Testament authors to read the Old Testament in a "spiritual" way that differed from the original meaning.