"The Weight of Glory" is the first essay in the book by the same name. After about a page and a half, I realized that this was the source of the oft-quoted "[We are] like an ignorant child who wants to go on making mud pies in a slum because he cannot imagine what is meant by the offer of a holiday at the sea." (Read the extended quote here.)
What I found particularly interesting was the point of application that he draws from contemplating the glory that we await, which surpasses all of the beauty and joy that we can imagine in this earthly existence. What are we to do with all of these thoughts of glory? We are to regard others in light of their own potential for glory.
"You have never talked to a mere mortal. Nations, cultures, art, civilisations - these are mortal, and their life is to ours as the life of a gnat. But it is immortals whom we joke with, work with, marry, snub and exploit - immortal horrors or everlasting speldours."(Read the rest of this quote in the second section here.)