I had a look at that essay or introduction by Lewis. It's well-written, of course. But I think his appeal to "mere Christianity" makes for something of a sentimental mirage. Whether non-historians read old books when they pick them up, or only confirm their prejudices as they read through their later, contemporary filters is another matter.

To understand the ancient assumptions, we have to set aside ours, as Lewis suggests, but that's easier said than done. That realization led to "postmodern" relativism, which is antithetical to Lewis's thesis about "mere Christianity." If each generation sees things differently, there's no bedrock philosophical or religious truth, but only a series of overblown "metanarratives."