if knowledge of christianity still exists then, only a few academic specialists would know anything about it from fragmentary evidence.

In Asia, there are ideologies, philosophies, and/or religions two- or threefold older than Christianity, and they still have hundreds of millions of followers, or, at least, more than 'only a few academic specialists' who know about them. In particular, thought from ancient Chinese and Indian civilizations still have great impact on the modern incarnations of those civilizations. Also, how evidence is gathered and stored is so much better than it was two thousand years ago. If ever a point comes that future civilizations look at ours as ancient, they will have information on our histories and cultures much better (in quality and quantity) than we have of, e.g., the origins of Hinduism, Judaism, or Mesopotamia.