Devising a completely objective, fool-proof methodology for separating the historical gospel material from the non-historical gospel material is obviously beyond the powers of the historical-critical method, but some attempts have still been able to come pretty close. One of the better attempts at devising a set of criteria for determining the historicity of Gospel material was that of J.P. Meier. He argued that five primary criteria could be used to determine how likely it was that facts or events in the gospels had a historical core. They are:
- Embarrassment: A fact or event that appears to cause embarrassment to the theology of the gospel authors is less likely to have been invented by them than a fact or event that bolsters their theology.
- Discontinuity: A fact or event that does not appear to have had any basis in earlier tradition is less likely to have been invented by the gospel authors than an event that may have been predicated in an earlier tradition.
- Multiple Attestation: A fact or event that appears to have been preserved down multiple lines of independent tradition is more likely to be true than one that is only preserved down a single line.
- Coherence: A fact or event that appears to be consistent with our present understanding of the historical context is more likely to be true than one which appears to be at odds with it.
- Rejection and Execution: A fact or event that looks as though it might provide an realistic explanation for the rejection or execution of Jesus is more likely to be true than the more tendentious explanations offered consciously by the gospel authors (e.g. divine providence, the Jews being in league with the devil etc.). (This criterion is less strong as it presumes historicity of the execution to begin with, but given that the execution of Jesus appears to satisfy each of the four previous criteria, it's based on a fairly solid foundation so far as second-order criteria go.)
Now there are very few facts or events in the gospels that appear to satisfy all of these criteria, which is why we can say very little with any certainty about the historical Jesus. However, a fact such as Jesus being from Nazareth - an otherwise, minor or incidental fact - appears to be one of those facts that we can state of Jesus most certainly because it does happen to satisfy each of these criteria:
- Embarrassment: The fact that Jesus came from Nazareth was inconvenient for those who accepted the OT prophecy that the Messiah was to be born in Bethlehem (Mic. 5:2). Matthew and Luke had to invent convoluted (and entirely contradictory) accounts to reconcile these two "facts" and even John appears to be aware of the theological difficulties concerning Jesus' origins in Nazareth (Jn. 7:41b-42).
- Discontinuity: There was no tradition in the OT - or any other Jewish literature - about the messiah hailing from Nazareth, or anything else of any significance concerning that town - in fact, the town of Nazareth does not appear once in any context in the OT. This discontinuity was plainly an embarrassment to Matthew who was required to invent a prophecy concerning Nazareth as a means of obscuring it (Mt. 2:23).
- Multiple attestation: This fact is recorded down at least four independent lines: Mark (Mk. 1:9), M (Mt. 2:23), L (Lk. 2:39) and John (Jn. 1:45-46).
- Coherence: It's consistent with our understanding of Jesus as an outsider, or a "Marginal Jew" in the words of J.P. Meier.
- Rejection and Execution: The fact that he came from such a backwards and inconsequential part of the world may have contributed to his being rejected as a messianic figure by many. John appears to suggest as much (Jn. 7:52).
Now again, none of this is foolproof, but it surely makes the competing suggestion - that the place of Jesus' birth was "made up" somewhere along the line, for want of a better expression - extremely difficult to accept.
In any case, what these criteria really allow us to do is make comparative probabilistic judgments concerning the historicity of various events in the gospels. Where Jesus' birthplace appears to satisfy all five of Meier's criteria, an event like the Last Supper appears to satisfy two (multiple attestation and coherence, with regards to Jesus' use of bread as a spiritual metaphor throughout the gospels) and fail two (it does not represent any kind of discontinuity because it appears to have predicates in both Jewish and Gentile tradition, and it fails the embarrassment criterion because the Eucharist is something that the early Christians would have been motivated to trace back to Jesus). At the other extreme we have plenty of events, such as the raising of Lazarus, that appear to fail all five criteria: there would have been a clear motivation for early Christians to invent stories like this, there are clear predicates in earlier traditions (e.g. Elijah), the story is presented down just one tradition (i.e. John), the story is at odds with any realistic historical context and it doesn't appear to offer any coherent reason as to why Jesus came to be rejected or executed by the Jewish authorities (cf. John 11:45-54).
Now none of this is to say that Jesus was definitely born in Nazareth, that he might have presided over the first Eucharist and that he definitely didn't raise Lazarus from the dead: we are incapable of making such definitive judgments. It is true, in one sense, to say that all content in the gospels may or may not be true: the mythicist is as justified in rejecting the Nazareth accounts as the fundamentalist is in accepting the Lazarus accounts. However, I think an objective methodological approach to the gospels - such as this one set out by Meier - can only lead us to the conclusion (for whatever it is worth) that the Nazareth accounts are far more probable than the Lazarus accounts, and that the Nazareth accounts are far more likely to find their origin in some historical memory than in the imagination of some early evangelist.
(Adapted from my RS post here.)