There are lots of things that we can know about the life of Jesus with a degree of confidence, his healing activity, his proclamation of the kingdom, his connection to John the Baptist, the call of disciples who continued the movement after his arrest and crucifixion, and so on. Beginning from this kind of secure information, one can produce a good sketch of the life of Jesus, and E. P. Sanders has illustrated how much one can do with this kind of data when we integrate them into an informed understanding of Jesus' historical context.
But knowing things about the historical Jesus is not the same as being able to write his biography. (The New Testament scholar Rudolf) Bultmann rightly pointed out that we do not have the data available to trace his psychological development in the manner of contemporary biography. Yet recent years have seen an increasing confidence in our ability to paint something approaching a complete picture of Jesus' life and personality, as if all the relevant and necessary materials for that complete picture are available somewhere. It just takes a bit of effort to get at them. We spend many painful hours sifting and honing criteria because we feel that the literary deposit is somewhere bound to contain all the material of real importance. Only matters peripheral to the task of reconstructing the key elements in his life have disappeared.
This kind of assumption develops out of an unrealistic perspective on the task. We proceed as if we are doing the work of restoration, clearing the dirt, the damage, the rust in order to unveil the real Jesus. But the quest is not about restoration. It is a task of ancient history and when understood as ancient history, discussion about the historical Jesus should constantly involve the reminder that massive amounts of key data must be missing.
It may be that we seldom reflect on this fact because the ideological investment in Jesus affects our historical research on him. Those ideological interests are, of course, many and varied, but the same kind of optimistic assumptions about the data set are shared by those from different ends of the spectrum, from those whose faith commitment compels them to regard the scriptural deposit as definitive, to those who look to a range of materials and methods in a bid to reconstruct a Jesus who is uncongenial to later Christian orthodoxy.
Let me illustrate the kind of thing I am talking about. According to almost everyone, one of the most certain things that we can know about the historical Jesus is that he was a disciple of John the Baptist. This is bedrock stuff and anyone familiar with Jesus research will know all about why. As it happens, I am inclined to agree with this; I suspect that Jesus did indeed have an association with John the Baptist and that it was important, in some way, in his development. But how important was John the Baptist, as an influence on Jesus, in comparison to other people? We know about the link between the two men because John the Baptist was himself famous -- Josephus devotes more time to him than he does to Jesus. So the tradition remembers and underlines the association between the two men.
But our influences are seldom solely other famous people. Perhaps the major influence on Jesus was his grandfather, whose fascination with Daniel 7 informed Jesus' apocalyptic mindset. Or perhaps it was Rabbi Matia in Capernaum who used to enjoy telling parables drawn from local agriculture. Or perhaps it was that crazy wandering Galilean exorcist Lebbaeus who used to talk about casting out demons by the Spirit of God. The fact is that we just don't know. We can't know. Our knowledge about the historical Jesus is always and inevitably partial. If we take the quest of the historical Jesus seriously as an aspect of ancient history, we have to admit that many of the key pieces must be missing.
The problem is that we are in denial. We simply do not want to admit that we do not have all the data we need to paint a complete picture of the historical Jesus. Good scholarship is sometimes born from a desire to fill in the gaps, and informed speculation can be a virtue. But over-confidence born out of an unrealistic expectation of the evidence will make future generations wonder what we were playing at.