What is the case against Gore Vidal's view that St. Paul "invented Christianity"? I was watching Gore Vidal being interviewed for CSpan's In Depth series and his book Live from Golgotha was being discussed. He claims that St. Paul "invented" Christianity. That Jesus came for the Jews only, but that St. Paul wanted to internationalize the appeal for Jesus and so twisted the truth. What are some arguments against this claim?
The case is vast, and, quite literally, fills volumes. I'm currently reading New Testament People God Volume 1: Christian Origins And The Question Of God, for example, which addresses this topic at length.

There are far too many arguments against this claim than can be addressed here (obviously), but some key points I have so far taken from the book mentioned above include:


We know that Christianity was a significant factor by the early second century, because we have strong evidence of persecution of Christians. The Roman empire, and various regions, had, to varying degrees, official policies relating to (and often forbidding) Christianity.
This means that Christianity had been around long enough to grow to a noticeable size. Even assuming an early date for the origin of Christianity (say immediately after Christ's death), this is an astonishing accomplishment for such a new religion in a world where new religions, new gods, and new practices, were a dime a dozen.  Pushing back a Christian genesis date makes this only that much more astonishing--not less (as most advocates of such a theory would have us believe)
An honest study of the content of the textual evidence (particularly, but not limited to the gospels and the rest of the New Testament) shows a progression in literary style as well as theology that would have taken many years to develop, and could not have been a "quick invention" by a single person let, alone by even a single generation.