In actuality, I believe that Lewis is correct on two levels:
1. Christianity allows us to "see" the world in a way that takes the random, arbitrary and meaningless aspects of it away and instead replaces these things with a cohesive explanation as to why we're here, how we got here, why the world and human nature is like it is and what the eventual solution to the world's problems shall be.
2. The only rational way to view reality is through the "lens" of God. The illumination of Christianity does not "exclude" anything except that which is false and contrary to the character of God. Non-believers see Christianity as restrictive - but it is only restrictive of that which is harmful, selfish, evil.
And, as far as your comment about how Christianity "sees...only in a particular way...[excluding] that [which is] beyond the scope of its illumination" - tell me: what way of seeing the world doesn't do this? What perspective doesn't exclude and "see" in a paticular fashion?







