In Matthew, Mark, Luke, and John, John the Baptist is someone who proclaims Jesus to be the Messiah. Is this historically accurate? And what criteria do biblical scholars use to address that question?
First, they compare the New Testament to non-Christian sources. The presumption is that the New Testament is biased, and so we should check out what non-Christian sources have to say. If sources without a Christian bias say some of the same things as Christian sources, then we can trust that the Christian sources are accurate on those points (unless a monk added some Christian stuff to the non-Christian source; in that case, scholars use other criteria to ferret out what's historical). And there's one first century C.E. non-Christian who discusses John the Baptist: the Jewish historian Flavius Josephus.
In Antiquities 18:116-119, Josephus mentions the destruction of Herod's army, and he says that many Jews felt this was a divine punishment because Herod had executed John the Baptist. According to Josephus, John was a just man who preached righteousness and practiced water baptism. And Josephus is clear that John had a huge following, which really frightened Herod. He states that Herod, "who feared lest the great influence John had over the people might put it into his power and inclination to raise a rebellion, (for they seemed ready to do anything he should advise), thought it best, by putting him to death, to prevent any mischief he might cause, and not bring himself into difficulties, by sparing a man who might make him repent of it when it should be too late" (William Whiston translation).
Josephus does not mention that John the Baptist proclaimed Jesus as the Messiah. What should we do with this? For some scholars, John did not make such a proclamation, but the early Christians claimed that he did because they viewed Jesus as the center of the universe. In this scenario, Josephus saw John as what he was: a righteous man who proclaimed repentance and managed to get a large following, and that's it. He wasn't a precursor to Jesus, as far as these scholars are concerned.
One could counter, however, that John proclaimed Jesus as the Messiah, but Josephus didn't see fit to record it. Maybe it didn't enter his mind, or he didn't view it as all that important. But one would think that John proclaiming the arrival of the Messiah would merit some discussion by Josephus. According to Josephus, Herod feared that John could cause sedition, and what better way would Josephus have to elaborate on that than to say that John proclaimed the arrival of the Messiah, the one who'd overthrow Herod and Roman rule in general? But we don't always write what others may expect us to write, plus the Gospel stories themselves don't say that Herod arrested John because of his proclamation of Jesus. So maybe Jesus was irrelevant to Josephus' discussion, which concerned Herod's execution of John.
And then there's the issue of bias. I one time heard a sermon by Armstrongite minister Fred Coulter that critiqued Judaism. According to Coulter, we can't always trust Josephus, for he was a Pharisee with his own set of biases. In Coulter's opinion, that's why Josephus doesn't mention Jesus all that much. He was one of those Pharisees who hated him! For Coulter, we need to read the New Testament to see what really happened. Many biblical scholars would probably respond that the New Testament itself is a biased source, but Coulter does well to point out that Josephus' ideology may color how he writes his history.
Second, biblical scholars may look at the New Testament itself to see if there are details that don't exactly mesh with Christian ideology. If a New Testament author presents a detail that's inconvenient to his religion, the argument runs, then it's most likely true. That's why many New Testament scholars (liberal and conservative alike) believe that the Gospels are correct when they affirm that John the Baptist baptized Jesus. In the Christian view, Jesus did not need John's baptism, for he was sinless, not to mention superior to John. But everyone seemed to know that Jesus had undergone John's baptism, and so the Christians felt a need to explain away this embarrassing detail (see Matthew 3:15).
As far as John the Baptist is concerned, there are details in Acts that can provoke some interesting questions. For example, Acts presents people who received John's baptism, yet they knew absolutely nothing about Jesus (Acts 18:24-25; 19:1-4). How could this be, if John proclaimed Jesus to be the Messiah? I once heard this one evangelical Bible study leader try to explain this passage, and he said that they got baptized by John, but they missed out on the session in which John discussed Jesus. Well, who knows? These characters were not natives of Judea, so perhaps they came to John, got baptized, and immediately left the country.
When I was at DePauw, one of my professors said that there was obviously tension between the John the Baptist and the Jesus movements, meaning that the John the Baptist movement didn't exactly fade away once Jesus came. The professor referred to Acts 19, which mentions followers of John the Baptist, who were around long after Jesus' (and John's) death. But he also said that the Gospels' repeated attempts to present Jesus as superior to John reflect a tension between the two movements. In this scenario, when the Gospel of John narrates John the Baptist saying that Jesus must increase, while he must decrease (John 3:30), it's saying: "Yo! John the Baptist followers, get a clue! You should be following Jesus now, not just sticking with the path that John the Baptist put you on. John proclaimed that Jesus was superior." But why were there still non-Christian John the Baptist followers, period, if John had obviously proclaimed that Jesus was the Messiah?
The Gospels present interesting details that may shed light on the issue. In the Gospels, John seemed to share Jesus primarily with his closest disciples, at least at first (see John 1:35-37). Jesus was not a big part of his public proclamation, which consisted of baptism, repentance, forgiveness, and the message that someone would soon come to execute God's wrath. Herod certainly did not think that John was proclaiming Jesus. Soon after Herod had executed John, he did not even know who Jesus was! He just heard reports that someone was doing all these miracles, and he thought it was John the Baptist risen from the dead (Matthew 14:2).
But, at some point, all sorts of people in Judea associated John the Baptist with Jesus. When the Pharisees asked Jesus by what authority he did what he did, Jesus responded, "Did the baptism of John come from heaven, or was it of human origin?" (Matthew 21:25, NRSV). That put the Pharisees in a quandary! They discussed amongst themselves, "If we say, 'From heaven,' he will say to us, 'Why then did you not believe him?' But if we say, 'Of human origin,' we are afraid of the crowd; for all regard John as a prophet' (vv 25-26)."
This discussion presumes that John the Baptist had publicly proclaimed Jesus to be the Messiah, at some point in time. The passage presents that as a no-brainer, something that everyone knew, including Jesus' opponents.
Is that historical? Would the Gospel writers present an untrue detail that opponents could so easily refute? If opponents or readers could just say, "Hey, that story can't be true! John didn't even proclaim Jesus," would the authors have included it?
That's a good question. One could ask about the Gospels' audiences, and whether or not they were familiar enough with John the Baptist to know one way or the other. The Gospels were in Greek, after all, which was not the dominant language of Judea (Aramaic was). Mark was written to Gentiles, for its author tries to explain to his audience Jewish customs that would have been familiar to Jews (Mark 7:3-4). Most scholars place the "Jewish Gospel," Matthew, in Antioch, the capital of Syria. So perhaps the Gospels' audiences were too detached from what happened in first century Judea to know what John the Baptist proclaimed.
Still, as I said above, the Gospel authors felt a need to explain why sinless Jesus underwent John's baptism. They couldn't just sweep that detail under the rug. They had to present Jesus as superior to John in light of this puzzling event, which somehow crossed people's radar screens. So perhaps their audiences knew more than one might think. And that may not be all that surprising, since news can travel outside of the country. People may have had relatives outside of Judea, or there were travelers who could carry the news.
Did John proclaim Jesus as the Messiah? It looks like a complex issue. In my opinion, one thing is certain: John told people to repent, and that softened their hearts for the arrival of Jesus. In that sense, John played a crucial role in the work of God.