Huxley's essay is about the difference between the material and the spiritual, and he uses the moon as an example. His point is that the two ways of thinking about the moon are not mutually exclusive, but reinforce one another.
Huxley begins by criticizing the philosophy of "nothing but," a kind of reductionist, materialist view of the world that he calls "mean as well as stupid." Huxley instead advances the "not only, but also" view of the world, in which the material and the spiritual coexist. His central example is Socrates, who was accused as heretical for believing that the moon was a stone. Socrates defended himself by arguing that "all men" know the moon is a god. While it may be empirically true to say that the moon is a rock, Huxley argues that art and poetry provide similar empirical evidence that the moon is divine.
Huxley next defines the nature of this "divinity." Central to his explanation is the term numinous, which refers to a "peculiar kind of feeling" or sense of the supernatural. Because humans have numinous feelings, it follows that there must be gods that inspire those feelings. Moonlight inspires a number of emotions: awe, despair, love. Even if we believe that the moon is "only" a stone, it inspires feelings; meditating on the moon can make one "feel most numinously a worm" and inspire a sense of man's smallness compared to the vastness of space.
Huxley concludes by arguing that man's ability to sense the numinous is evidence of his inherent spirituality and connection to a kind of higher, transcendent reality.