There is an assumption at work in a lot of historical Jesus research that all the relevant and necessary materials for a reasonably complete picture of Jesus are available. They are available somewhere and we can get at them somehow. We just have to work hard to get to 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. We speak of what we can say about the historical Jesus "with confidence" because we are sure that the really key data has to be present. Only matters peripheral to the task of reconstructing the key elements in his life has disappeared.
The 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 about 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 stakes in so major a figure inevitably interact with 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, don't we?