I'm going to quote this one at length. It is from the same atheist author I linked to in my previous post. It is part of a longer explanation as to why essentially no serious historians of the first century Middle East, be they Christian, Jew or atheist, think that Jesus was a made-up figure. But it is also a very good example of how historical reasoning works.
But probably the best example of an element in the story which was so awkward for the early Christians that it simply has to be historical is the crucifixion. The idea of a Messiah who dies was totally unheard of and utterly alien to any Jewish tradition prior to the beginning of Christianity, but the idea of a Messiah who was crucified was not only bizarre, it was absurd. According to Jewish tradition, anyone who was "hanged on a tree" was to be considered accursed by Yahweh and this was one of the reasons crucifixion was considered particularly abhorrent to Jews. The concept of a crucified Messiah, therefore, was totally bizarre and absurd.
It was equally weird to non-Jews. Crucifixion was considered the most shameful and abhorrent of deaths, so much so that one of the privileges of Roman citizenship is that citizens could never be crucified. The idea of a crucified god, therefore, was absurd and bizarre. This was so much the case that the early Christians avoided any depictions of Jesus on the cross - the first depictions of the Crucifixion appear in the Fourth Century, after Christian emperors banned crucifixion and it began to lose its stigma. It's significant that the earliest depiction of the crucifixion of Jesus that we have is a graffito from Rome showing a man worshipping a crucified figure with the head of a donkey with the mocking caption "Alexamenos worships his god". The idea of a crucified god was, quite literally, ridiculous. Paul acknowledges how absurd the idea of a crucified Messiah was in 1Cor 1:23, where he says it "is a stumbling block to the Jews and an absurdity to the gentiles".
The accounts of Jesus' crucifixion in the gospels also show how awkward the nature of their Messiah's death was for the earliest Christians. They are all full of references to texts in the Old Testament as ways of demonstrating that, far from being an absurdity, this was what was supposed to happen to the Messiah. But none of the texts used were considered prophecies of the Messiah before Christianity came along and some of them are highly forced. The "suffering servant" passages in Isaiah 53 are pressed into service as "prophecies" of the crucifixion, since they depict a figure being falsely accused, rejected and given up to be "pierced .... as a guilt offering". But the gospels don't reference other parts of the same passage which don't fit their story at all, such as where it is said this figure will "prolong his days and look upon his offspring".
Clearly the gospel writers were going to some effort to find some kind of scriptural basis for this rather awkward death for their group's leader, one that let them maintain their belief that he was the Messiah. Again, this makes most sense if there was a historical Jesus and he was crucified, leaving his followers with this awkward problem. If there was no historical Jesus at all, it becomes very difficult to explain where this bizarre, unprecedented and awkwardly inconvenient element in the story comes from. It's hard to see why anyone would invent the idea of a crucified Messiah and create these problems. And given that there was no precedent for a crucified Messiah, it's almost impossible to see this idea evolving out of earlier Jewish traditions. The most logical explanation is that it's in the story, despite its vast awkwardness, because it happened.