Religion v. myth?

One line Lewis' writes does not make sense to me "I suspect that men have sometimes derived more spiritual sustenance from myths they did not believe than from the religion they professed." (Lewis, Myth Became Fact) I understand the point he is trying to make in context of his writing. His friend Cornelius is basically saying religious people can be a bit soft or weak. He appears to be rolling with that idea and saying that if a person is not following a set of principles within a religion at all but still calls them self religious, can they get anything at all out of that. Where maybe someone who has not been exposed to religion can hear a myth and be more edified in that then the first person. Granted I understand that Lewis' mentions 'sometimes', I still read this and really just can't see it. If we use Christianity, let's look at a very sinful man who lives a life of partying and not going to church. When talking about religion though he claims to be Christian. Well is he getting much out of that religion? No... yet maybe he can. If he calls himself Christian and has been properly exposed to Christian beliefs. Within those Christian beliefs he can come to find truth and faith, if he were to hit a low point and turn to the faith he claims. Now if a person who does not have much religious exposure hears a story like Narina. That is an incredible story to them, but are they edified if they don't understand or have an explanation about what the story represents. What would that mean to them? It can't be spiritually fruitful without an understanding. I can't understand but this is a new idea, the idea of myth that is.