“Can’t you lead a good life without believing in Christianity?”
The answer to this depends to the answer of this other question, “Are you a man or a rabbit?”
If one is a man, an actual human being, then one would naturally thirst for knowledge. It’s the job of man to try to find out the truth of life. A rabbit, on the other hand, all it does is munch grass and be cute. It does not care (at least not consciously) about how a grass grows or how it provides it energy. It simply needs to know if it fills its stomach or not, if it’s useful to it or not.
When someone asks “Can’t you lead a good life without believing in Christianity?” Lewis writes that it sounds more like “Is Christianity useful?” The question is one that does not express concern for truth itself but what would come out of it if it’s true.
Apart from the intentions of asking this question, the answer to the first question is no. Certainly one could have leaded a good life with honest ignorance about Christ. There are quite a few men like this. Lewis offers the reader some names: Socrates and Confucius among others. However, it is different to know about the existence of Christ and decide to evade it. Christianity is a choice in this case. And rejecting it despite knowing its truth would be just like “the man who won’t go to the doctor when he firsts feels a mysterious pain, because he is afraid of what the doctor may tell him.” In the end because of cowardice, the man is doomed.
To the second question about usefulness: yes, Christianity is “useful.” It brings salvation. But becoming a Christian because of its usefulness will not make one a true Christian. What that person wants is not salvation from Christ, it’s to avoid hell. These two are very different goals. It doesn’t really matter if it’s useful or not, what should matter is if it’s true or not. Christianity “gives an account of facts.” And I can tell you that Christianity is very true indeed. Like C.S Lewis wrote, I also “believe in Christianity as I believe that the sun has risen: not only because I see it, but because by it I see everything else.”
This whole topic reminded me of the last book of the Chronicles of Narnia. There is a scene where a soldier, despite being Carlomene and not Narnian, is accepted by Aslan because he withheld the morality held by Narnians; mainly courage and loyalty. He was saved not only because he had led a good life but because he had an honest ignorance about Aslan since he had grown up in another country that knew nothing of him.
There is a whole other point to this argument though. No one can truly lead a good life without Christ. It simply can’t be possible. If one does, but had put “a good life” as a goal, then one would had missed “the very point of [one’s] existence.” Everyone’s life was created for God and only for God. Where is the point of living a good life when one has no one to offer it to?