I'm not a Christian, so I can't really comment on it that much. However CS Lewis was speaking in a fever. He became a convert. And that's what happened. Christianity offered something to him and he went for it. It could have been any religion, or any psychology for that matter. A religion offered him something, the result of which is a instant true believer. You don't have to be a true believer. You can be an agnostic or an atheist, or anything in between. Whatever works for you. So I never dwell on religion or lack of one.