The dog eagerly scampered down the sidewalk, followed by his master holding his leash. The dog sniffed all the way around a sign-post, then tried to continue on ahead, not realizing he’d looped his leash around the post. His master gently pulled back on the leash, trying to get his dog to retrace his steps and get the leash unsnarled. The dog strained forward with all its might, wanting to move on ahead and not understanding why he was being asked to halt.
Now this was a dutiful dog, and he soon realized he must submit his will to his master, even though it made no sense to him. If the dog had any theological training, he might have even labeled his desire to go forward as a “sin,” and to yield himself to the inscrutable will of his master. The owner, if he knew how, might wish to expose this as false dogma, to explain that he too wanted to continue moving forward. He might add that he had to deny his dog’s will to pull forward on the leash in order to grant him his true wish, to resume their walk together.
Prior summary is from a C.S. Lewis Letter to his friend Arthur Greeves a few years after his conversion to Christianity. (Based on Collected Letters of CSL, Vol 2, 122):
http://booksbycslewis.blogspot.com/2008/11/parables-for-pilgrims.html
Please read 85% of the letter itself here: http://tinyurl.com/82q8j3z