This section would end the investigative part of the first chapter of The Essential Jesus, titled "The Baptism of John." I probably won't blog the last part of the chapter, "Life Reflections."
We can speculate further about the nature of John the baptizer's message by looking at what became of those who followed his teaching, in the decades long after he was beheaded. We hear about such followers at Ephesus in Acts 18-19. The first is Apollos in Acts 18:24-28. He is instructed in the "Way of the Lord" and proclaimed the coming of Jesus, but was only aware of the baptism of John.
This scenario only makes sense if John the Baptist proclaimed the way of the Lord, if this expression goes back to him. In making this connection, it is interesting that both Isaiah 40 and Malachi 3 use the word "way." These are again the passages that the gospels remember in relation to John's prophetic ministry. So it is not a stretch to say that John must have proclaimed quite literally "the Way of the Lord" as one of his key messages.
Followers of the Way were thus individuals who bought into his message--whether they believed in Jesus or not. Apollos could be a follower of the Way and know very little about Jesus. What he knew was John the baptizer's prediction that the "anointed one," the messiah, was coming. The Gospels also remember this message as part of what John proclaimed: "The one who is more powerful than I is coming after me; I am not worthy to stoop down and untie the thong of his sandals" (Mark 1:7, NRSV).
Acts 19:1-7 reinforces this interpretation. Paul finds certain "disciples" at Ephesus. These individuals only know the baptism of John. They do not seem to know much of anything about Jesus. When Acts calls them "disciples," it must not mean followers of Jesus but followers of the Way, followers of the movement started by John the Baptist.
For Acts, receiving the Spirit is the big distinction between the two movements. The Jesus movement was part of the Baptist movement. Both were followers of the Way. But the Jesus movement believed Jesus was the messiah John predicted, and it involved the Holy Spirit. Jesus' followers received the Holy Spirit. At some point, John's baptism became distinguished from baptism "in the name of Jesus," so much so that Paul has the followers of the Baptist get re-baptized so they will receive the Holy Spirit in Acts 19.
The Gospel of John indirectly supports and extends this interpretation. None of the gospels are simply recording history. They each present Jesus in a way that brings out particular themes and emphases.  One of the intriguing features of the Gospel of John is the extent to which it downplays John the Baptist in relation to Jesus. John never actually mentions the Baptist baptizing Jesus. John's presentation makes it very clear that the Baptist's mission is basically over once Jesus arrives (e.g., 1:36-37; 3:30). Only John's gospel tells of Jesus' followers baptizing at the same time as the Baptist. Finally, in contrast to Matthew 11:14, the Gospel of John denies that John the Baptist is Elijah (1:21).
Why would the Gospel of John downplay John the Baptist so much more than the other gospels? A plausible answer is that, as we see in Acts 18-19, there were followers of John the Baptist at Ephesus who not only were not Jesus followers. There may have been followers of the Way there who opposed the Jesus movement, who opposed the idea that Jesus was the messiah. John and Acts are thus written in such a way as to make it clear that Jesus is the one John predicted. 
So not only did John proclaim the coming judgment of God and the potential restoration of political Israel. He was one of those Jews who also predicted the coming of a king to rule Israel in this coming kingdom, the messiah.  In preparation, he called Israel to repent and to wash themselves in the Jordan, symbolizing the washing and forgiveness of their sins.
What a massive movement he must have started! We do not know much about it apart from those of his followers who went on to believe Jesus was in fact the predicted messiah. But at the time his movement must have paralleled that of Jesus, perhaps even been bigger. Some in this movement of the Way may have known very little about Jesus at all. It must not have been clear at the time that John endorsed Jesus as the one he predicted.
In fact, we can read passages like Matthew 11:2-6 as the Baptist himself having some uncertainty. John is in prison but sends some of his followers to ask Jesus if he is the one. The things he is doing make him a prime candidate. But John must not have been entirely certain. 
What is clear is that all four gospels see Jesus' ministry in continuity with that of the Baptist. In Matthew, Mark, and Luke, Jesus is baptized by John, which indicates that Jesus endorses John's message. As many Jesus scholars have argued, this fact has enormous implications for how we understand Jesus' own mission and message.  We must understand Jesus' words not as wise sayings a philosopher might say but as words said against this historical background.
[text box on the Jesus Seminar]
 As we will see more clearly in the second volume of this series.
 The incident in Acts 19 is thus not about entire sanctification (as many holiness revivalists preached in the late 1800s/early 1900s). Still less is it about the necessity of tongues as evidence of truly being converted (certain Pentecostal groups). Acts 19, including its added evidence of tongues, was originally a polemic against followers of the Baptist who did not believe Jesus was the messiah.
 Again, if we have to pick a Jewish group his message best fits, it would be the Essenes.
 This scene has of course given rise to much speculation as to John's motives, especially if you bring Luke 1 and John 1 into the conversation.
 One of the first to point out this fact was A. E. Harvey, Jesus and the Constraints of History **. E. P. Sanders' Jesus and Judaism then built on this approach.