This, Lewis's "third point" in the chapter, refers back to his first two points, namely
(1) "Humanity has been making some big mistake. We are on the wrong road" (page 29).
(2) "The Being behind the universe is intensely interested in right conduct. . . . If there does exist an absolute goodness it must hate most of what we do" (pages 30, 31).
He's attempting to get at the truths taught in the Psalms and Romans regarding the depravity of man.
Psalm 14:2-3 (cf. Psalm 53:2-3; Romans 3:10): The LORD looks down from heaven on the children of man, to see if there are any who understand, who seek after God. They have all turned aside; together they have become corrupt; there is none who does good, not even one.
Romans 3:23: All have sinned and fall short of the glory of God.
If the reader rejects these truths, he is rejecting the foundational elements of Christianity. "It is after you have realized that there is a real Moral Law, and a Power behind the law, and that you have broken that law and put yourself wrong with that Power--it is after all this, and not a moment sooner, that Christianity begins to talk" (p. 31).