James Sennett's chapter in The Chronicles of Narnia and Philosophy considers several views on the extent of salvation:
Universalism: Everyone will be saved.
Pluralism: There is no one, true religion. Multiple religions are legitimate paths to God.
Inclusivism: There is one, true religion, but some who are technically in other religions are nonetheless on a legitimate path to God by means of the correct religion, even if they don't know it.
Exclusivism: There is one, true religion, and the only path to God is through explicitly following that religion.
Sennett argues, correctly I think, that Lewis was an inclusivist. He allows for Emeth to be saved without any explicit trust in Aslan, but he insists that Emeth was following Aslan while falsely believing he was following Tash. Aslan clearly states that Aslan and Tash are not the same being, and the followers of Tash are evil and do not make it into Aslan's country. Universalism and pluralism are as easily ruled out as exclusivism. I haven't spent an awful lot of time thinking about inclusivism, because it seems so hard to square with Paul's train of thought in Romans 10. But Sennett has helped me see that Lewis' inclusivism makes sense of one puzzling element of the Narnia stories, and he's also helped me think a little more fully about what an inclusivist view should look like.
Sennett argues that inclusivism best explains something that might otherwise be puzzling in the Narnia stories. See The Mouse Trap Theory of Atonement at Green Baggins for a serious discussion of Lewis' theory of the atonement in the Narnia books. After reading Sennett, I'm now wondering if the discussion makes any sense. It's an attempt to get an entire theory of atonement out of an event that isn't really atonement for anyone but Edmund. Sennett has a much better alternative. He insists that the Narnians' following of Aslan is not Christianity. You don't have anything in Narnia like salvation by means of faith in a work of atonement. The stone table was one event for one person that turned the tables in one war against one opponent. It's much better to think of Narnians who follow Aslan in a way more like how Christians generally see faithful Jews before the time of Christ and how Lewis saw Emeth following Aslan without knowing it when he thought he was serving Tash. I think what Sennett is suggesting is that the real atonement for Narnians is the same one for us, namely the cross in our world. The Narnians don't know this to put explicit faith in it, but it's enough that Aslan does when he initiates the work of faith in their lives to guide them along in their progress toward greater understanding, some of which may only come after their death (as was the case with Emeth). I think this makes much better sense of what Lewis is doing with the stone table and how he might say that Narnians are saved.
Anther intriguing statement Sennett makes is that Aslan is not Jesus. I thought it was obvious that Aslan is Jesus. Isn't the stone table supposed to refer to the cross, even if it isn't really salvation for all the Narnians? Well, yes, literarily. But in the world of the fiction, Aslan is a lion. Jesus is a man. The incarnation of the first person of the Trinity as a man in our world is not the same incarnation as his incarnation as a lion in the Narnian world. The incarnation is hard enough to figure out philosophically, but a double incarnation? Fortunately, Prosblogion has already had two discussions of that issue for those who are curious.
Finally, it occurs to me that inclusivism fits best with a Calvinist model of divine sovereignty. Sennett's way of describing who among other religions is genuinely on the path to salvation is that they're the ones God is working in to move them toward the right attitudes and practices, despite not having the right information to know what the gospel even is. Without that, and without the evidence of explicit faith in Jesus Christ, it's very hard for there to be objective criteria for someone to be saved. The easiest way around that is for the criteria to be simply whoever God is genuinely working in, a work that will always be brought to completion, but that requires Calvinist views of divine sovereignty over human salvation. There may be other ways to do it, but that's certainly the easiest answer to the problem. Ironically, Calvinists are probably more likely to be opposed to inclusivism than other groups, and inclusivists rarely want to be Calvinists.