One reason he believes this is because the gospels cannot be trusted. They contain so much that is obviously made up that even people like C.S. Lewis and William Lane Craig admit not everything in them is historically accurate. As a result, Callahan pretty much dismisses anything found in them.
But the untrustworthiness of the gospels doesn’t mean one cannot extract valuable information from them. In fact, they contain clear evidence that Jesus existed – evidence that of course also adds to what we can know about him.
To begin with, there are clues within the gospels themselves that they had precursors – earlier writings on which they were at least partially based. But this means that some writings were already around not very long after Jesus's death. Moreover, there appear to have been several of them (Luke in fact refers to “many” such works), and yet they all seem to agree on certain basic points, such as that Jesus was crucified by Pontius Pilate at the instigation of the Jewish authorities. This is therefore pretty strong evidence that at least these parts of the Jesus story are true.
In the second place, there are several things in the gospels that were problematic for Jesus’s early followers and thus are very unlikely to have been invented. The most obvious is the crucifixion itself (mentioned not only in the gospels but in Paul’s letters as well). Why would a Jewish cult have made up such an event given that it goes completely against what was expected of the Messiah? It just doesn’t fit. A far more plausible story is that there was an actual execution of someone regarded as the Messiah, and that his followers then had to adjust their messianic expectations.
Another clue found in the gospels is that Jesus was said to have come from Nazareth. Again, this goes against what was expected: the Messiah was supposed to be from David’s city of Bethlehem. It is for this reason that fictitious stories are told about his birth in Matthew and Luke, so as to explain the problem away. (We know they are fictitious because they are utterly implausible and because they are inconsistent with one another.) But if Jesus was an invention, why would he have been said to be from the wrong place to begin with?
Related to this is another clue that I find very interesting. It is claimed that in his hometown of Nazareth, Jesus was almost entirely unable to perform miracles. This is telling, because, first, it is unlikely his followers would have invented a story of failure about their leader; and second, because it fits in with what we know about human gullibility. The people in his hometown knew him, they had seen him grow up, and thus they were much less disposed to accept the idea that this individual they had known all their lives was suddenly a great miracle worker. The only part of the story that is clearly invented is the explanation offered to justify the unexpected failure: that it was due to the lack of faith of the Nazarenes. By putting the blame on them – and making a theological point in the process – the one who came up with this explanation found a pretty good excuse. (And it might very well have been Jesus himself who first explained the failure in this way.)
The last problem I’ll mention is Jesus’s baptism. Again, this is unlikely to be an invention. The purpose of baptism is to cleanse away one’s sins, and as many critics have pointed out, it seems Jesus wouldn’t have needed to do that. Moreover, the story suggests that Jesus was originally a follower of John the Baptist – an idea that finds further backing in the fact that Jesus had essentially the same apocalyptic views as John, and also in the fact that the story of the baptism itself changed over the years so as to make Jesus less and less like a follower and John less and less like the leader. The baptism is therefore something that in all probability occurred, and which Christians later had to somehow deal with so as to make it fit with their theology.
The gospels, then, do present pretty solid evidence that there was a historical Jesus. Furthermore, they present various facts about him that are entirely believable. The Jesus of history, it seems, was not as unrelated to the Jesus of scripture as Callahan suggests.