The Lion, the Witch and the Wardrobe (Chronicles of Narnia, #2) - C.S. Lewis, Pauline Baynes

"You have forgotten to clean your sword," said Aslan.
It was true. Peter blushed when he looked a the bright blade and saw it all smeared with the Wolf's hair and blood. He stooped down and wiped it quite clean on the grass, and then wiped it quite dry on his coat.
"Hand it to me and kneel, Son of Adam," said Aslan. And when Peter had done so he struck him with the flat of the blade and said, "Rise up, Sir Peter Wolf's-Bane. And, whatever happens, never forget to wipe your sword."

What do you suppose is the significance of wiping the blade? Or more specifically, the 'never forget' part?