For instance (ok, someone went and pushed my LECTURE button), Jesus almost certainly did exist (or at least someone named Yeshua around 0-30 CE plus or minus a few years almost certainly formed the basis for christianity) but he was neither a singular figure nor did he preach anything unique. The message commonly ascribed to Jesus was part of the reformism of judaism that had been sweeping through Israel starting about 160 BCE when the Hellenistic Greeks took over. There were in fact any number of wandering 'holy men' preaching the same set of messages at the same time. It's kind of ironic that what xians are always selling as 'christian values' were actually acquired a century or more before Jesus' birth from a bunch of polytheistic pagans. That's right, folks -- christian values are really good old fashioned greek philosophical values.
Now, how do you think that's gonna fly with your average fundie?