There are a couple of things. First, there's the radical dissimilarity between what Christians claimed about Jesus' resurrection and the available pagan options of the day. Pagans routinely mocked Christians for their belief in Jesus' resurrection for much the same reason that members of the modern Jesus Seminar do: we all know that dead men stay dead. Besides, in the ancient pagan worldview, resurrection was not only impossible but also undesirable. Why would anyone want to revisit this world of toil and pain? Much better to seek immortality through great deeds (the Homeric vision). This belief about the resurrection of Jesus is absolutely crucial in understanding the early Christian movement and its beliefs. Their belief in resurrection has no pagan precursors. There is no natural development from any pagan philosophy or religious perspective that we know about and the Christian vision of Jesus' (and our) post-mortem existence.
Second, there is also the fact that pagans routinely resisted Christianity, sometimes with violence. If the parallels between Christianity and paganism were so many and so obvious as is claimed by modern revisionists, we wouldn't expect such vehement opposition.
Third, in defending their gospel, Christians did not routinely resort to pagan sources, but rather attempted to demonstrate that the Jewish bible was as good as or better than Plato et al. Again, if Christianity were so similar to paganism and indeed was simply a development of it, why would Christians appeal to the holy books of a weird sect based in Palestine that had a reputation for thumbing their noses at pagans?
Essentially, the claim that Christianity is basically paganism in disguise simply cannot be held with any intellectual integrity when the actual history is examined. If Christianity were another version of paganism, its earliest proponents and opponents obviously (and universally) did a very bad job of understanding it.
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