This is not the first time in Mere Christianity that Lewis has touched on his controversial belief in inclusivism. See the second Bookmark for page 64 for a discussion of this topic.
John 14:6 (as well as other passages) would, at the very least, cause one to doubt Lewis's proposal here ("I am the way, and the truth, and the life. No one comes to the Father except through me"). See also John 10:14 (ESV), quoting Christ: "I am the good shepherd. I know my own and my own know me."
Inclusivism necessitates a kind of salvation by works--the possibility of which Lewis has denied on page 142 (see Bookmark): "If there was any idea of a sort of bargain--any idea that we could perform our side of the contract and thus put God in our debt so that it was up to Him, in mere justice, to perform His side--that has to be wiped out."
In short, nowhere does the Bible validate Lewis's view that people who believe in religions other than Christianity can "belong to Christ without knowing it."