"It's like this," Enola began. "You and Harry recently established a Bond of Matrimony, powered by love. But what Tom Riddle did was to use his hatred of Harry, the world, himself, who knows, to power the Killing Curse that he used. But James and Lily were ready for that, and used their love for Harry ... and ultimately, their lives ... to make sure that for all that Riddle could hurt Harry he could never kill him with magic. They used their love as a shield, and they literally brought an aspect of the prophecy to fulfilment when they did that ... by making sure it was now Harry who could do the killing, and not Riddle.