Take his approach to objections to Christianity based on the makeup of the universe:
If the universe is teeming with life other than ours, then this, we are told, makes it quite ridiculous to believe that God should be so concerned with the human race as to 'come down from Heaven' and be made man for its redemption. If, on the other hand, our planet is really unique in harbouring organic life, then this is thought to prove that life is only an accidental by-product in the universe and so again to disprove our religion.... This kind of objection to the Christian faith is not really based on the observed nature of the actual universe at all. You can make it without waiting to find out what the universe is like, for it will fit any kind of universe we choose to imagine.
This is true enough in itself, but it is an incomplete truth: it's bleeding obvious to someone not invested in Lewis's argument that it could just as easily be deployed against him. If the universe was empty but for the sun and Earth, that would be prime evidence that we are God's prized creation. Since it is in fact teeming with uninhabited celestial bodies, Earth's harbouring of (sentient) life is taken to be proof of God's existence. Lewis doesn't himself argue this--he's more concerned with Reason as a manifestation of God rather than the centrality of man to creation--but his logic is very similar: man is special, and can only be so because of God.
Lewis goes on to attack anti-theist arguments that deal with the universe's size:
Medieval thinkers believed that the stars must be somehow superior to the Earth because they looked bright and it did not. Moderns think that the Galaxy ought to be more important than the Earth because it is bigger. Both states of mind can produce good poetry. Both can supply mental pictures which rouse very respectable emotions--emotions of awe, humility, or exhilaration. But taken as serious philosophical argument both are ridiculous. The atheist's argument from size is, in fact, an instance of just that picture-thinking to which, as we shall see in a later chapter, the Christian is not committed.
The problem is the atheist argument from size is in fact a straw man. No one thinks galaxies are "more" important than the Earth. Materialism dictates that cosmically, nothing is more important than anything else. It just is.
Lewis gets this very wrong indeed, but like a student who flubs a sign early in a mathematical formula and sends the rest of his calculations far astray, he ought be given credit for having done the work anyway.