How do historians test what Jesus really said, and do the tests work?

They use a set of tests called the criteria of authenticity, and scholars now argue about how much weight the tests can bear. The quest for the historical Jesus consists of academic efforts to determine what words and actions, if any, may be attributed to Jesus. By 1987 D. Polkow listed 25 separate criteria in use. The best known are multiple attestation, dissimilarity and embarrassment.

Multiple attestation, first developed by F. C. Burkitt in 1906, assumes that the more sources report an event or saying, the more likely it is historically accurate. Dissimilarity, introduced by Ernst Käsemann, compares a gospel passage to the Jewish context of the time and gives weight to what is dissimilar. The "Son of Man" shows how it works: it is Jesus' most common self-designation in the Gospels, yet none of the New Testament epistles use the expression, nor is there evidence that the disciples or the early church did.

Embarrassment rests on the assumption that the early church would not have gone out of its way to create or falsify material that only embarrassed its author or weakened its position. The crucifixion fits: the method of execution was considered the most shameful and degrading in the Roman world, so followers are unlikely to have invented it.

But the tests have limits. Clear-cut cases of embarrassment are few, and what embarrasses in one era and social context may not in another. Chris Keith argues that the criteria are literary tools, indebted to form criticism, not historiographic tools, and were meant to discern pre-Gospel traditions, not to identify historical facts.