Didn’t the simple fact that he had taken a Hallow, and turned it into a Horcrux, demonstrate that he did not know this last great Wizarding secret?
She sat beside her sister, as unlike her in looks, with her dark hair and heavily lidded eyes, as she was in bearing and demeanor; where Narcissa sat rigid and impassive, Bellatrix leaned toward Voldemort, for mere words could not demonstrate her longing for closeness.
A couple of them rose from their chairs to demonstrate their willingness for immediate action.
Who has volunteered to demonstrate what happens to those who continue to fight when the battle is lost?
"Neville here is now going to demonstrate what happens to anyone foolish enough to continue to oppose me," said Voldemort, and with a flick of his wand, he caused the Sorting Hat to burst into flames.
There are no more uses of "demonstrate" in the book.