From a piece he wrote critiquing his own play, Magic:
Magic is a wonderful little comedy that I read some years ago during a bout of depression. It deals with faith, reason, and skepticism on the scale of ordinary life. The most dramatic thing that happens in this play, in which characters furiously debate whether magic is, in fact, real, is a lamp turning on.
With characteristically Chestertonian wit and humor, Magic insists on faith and reason, rather than faith or reason, and dramatizes the impoverishment of humanity when the two are opposed. But it's not a straight allegory or morality play; Chesterton leaves things ambiguous, including the very subject of the play.
If it's so ambiguous, then what's the point? you might want to ask. I don't know, but Magic was exactly what I needed when I read it, the same way many people claimed to have been saved from madness by Chesterton's Man Who Was Thursday. To quote his introduction to the book of Job, in reference to God's refusal to answer Job's questions:
The other great fact which, taken together with this one, makes the whole work religious instead of merely philosophical is that other great surprise which makes Job suddenly satisfied with the mere presentation of something impenetrable. Verbally speaking the enigmas of Jehovah seem darker and more desolate than the enigmas of Job; yet Job was comfortless before the speech of Jehovah and is comforted after it. He has been told nothing, but he feels the terrible and tingling atmosphere of something which is too good to be told. The refusal of God to explain His design is itself a burning hint of His design. The riddles of God are more satisfying than the solutions of man.
Magic has been revived a few times in out of the way places by fellow devotees of Chesterton. (Here's a review of a production from about the time I read the play.) It's never been staged anywhere close enough for me to see it performed, but I hope that will change someday.
In the mean time, do check Magic out. It's a short three act play; you can easily read it through in one sitting. It's available free at Project Gutenberg and in volume 11 of The Collected Works of G.K. Chesterton, which is still a work in progress (at 37 volumes!) from Ignatius Press.