Here Lewis breezes past another highly controversial subject: inclusivism--the idea that people may be Christians and not know it.
Probably the prime example of this belief in Lewis's works is the character of Emeth in The Last Battle. Emeth ends up, to his surprise, being welcomed to Aslan's Country after his death in spite of the fact that he had been a servant of Tash (who represents the Antichrist) in life. Aslan (a Christ figure) explains, "I take to me the services which thou hast done to Tash. For I and he are of such different kinds that no service which is vile can be done to me, and none which is not vile can be done to him."
This idea is at best highly suspect, not being taught in Scripture. Would Lewis have believed that someone could be a Muslim or a Hindu or an atheist and not know it? Aslan's statement above at the very least implies that people are saved by their works, a fact which Lewis denies on page 63 of Mere Christianity: "But even the best Christian that ever lived is not acting on his own steam--he is only nourishing or protecting a life he could never have acquired by his own efforts." See also Ephesians 2:8-9: "For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast." See also the passages below.
John 14:6 (ESV): "Jesus said to him, 'I am the way, and the truth, and the life; no one comes to the Father but through Me.'"
John 10:14 (ESV), quoting Christ: "I am the good shepherd. I know my own and my own know me."