What Does That Mean for Non-Christians?

CS Lewis believes that we encounter God as the divine law giver, that there's the law of human nature. He says it's the foundation of all clear thinking about ourselves and the universe we live in. And I mean, what does that mean for non-Christians? Because I suppose it's one thing to tell Christians that their sense of morality comes from the God that they believe in. But how would you approach that with non-Christists?