A well-known
literary critic and
theologian who has a presence in
geek culture because of his 7-book
Chronicles of Narnia (a course on Christian theology presented as a series of children's fantasy novels) and, to a lesser extent, the Space Trilogy. Lewis wrote a series of popular
apologies, or defenses of Christianity, that have been enormously influential in the English-speaking world.
Lewis had a knack for relating complex theological ideas to feelings and experiences that were shared by believers and non-believers alike--for example, he begins The Case for Christianity with the anger you would feel if another person stole your seat on a bus. The argument itself has some flaws, but it illustrates Lewis' strength as an apologist: he begins with experience rather than intellectual abstractions or psuedo-logical arguments. His appeal lies also in his great honesty. In a passage from his autobiography, Surprised by Joy, he writes of his experience in an English public school where each of the older boys took on a younger boy as a lover. Lewis explains that he doesn't criticize this practice for two reasons: first, that it was the closest thing to a human relationship that most boys had in the school; second, that pedophilia is one of two sins that he was never tempted to engage in (the other is excessive gambling) and so he doesn't feel qualified to comment on it. He then says, "if you ask, 'does this mean that the sins you do talk about...' the answer is yes."
How can you not like this guy? As a result of this and his other writings (not to mention his biting wit, exhibited most amusingly in the Screwtape Letters, he has become an important influence on Christian writers from every denomination.
A side note: Lewis was a friend of J.R.R. Tolkien, who was an important influence on his decision to become a Christian. However, they later grew rather distant--in part because Tolkien was Catholic and was upset that Lewis had become Anglican, and in part because Tolkien was a bit jealous of Lewis' success as a theologian.