Now, I'm not claiming that Harry Potter's world is an integrated and purposely-thought-out Christian allegory, like Narnia. Neither is it a coherent mythical world whose author is so steeped in Christianity that everything is viewed through this lens, like the worlds Tolkien invented. But neither is Hogwarts a secular adventure, where evil is defined as material harm to others. Voldemort is evil not just because he has caused mayhem, or killed people. He is evil because he has deliberately torn asunder something within himself that was created to stay whole.
This revelation of what, in the wizarding world, constitutes the ultimate -- yes, I will say sin, although Rowling does not use the word -- comes while Dumbledore and Harry are pursuing information about Voldemort's past through means of the Pensieve. This device, to which we were introduced in Prisoner of Azkaban (and which is, by the way, a tempting object for any wife whose husband insists on contradicting her based on his own obviously faulty recollections) enables third parties to enter a virtual reality of another's memories. Dumbledore has gone to great effort to obtain memories of those who surrounded Tom Riddle, the future Voldemort, in his youth, in an attempt to identify his weaknesses and so defeat him. A breakthrough comes when they obtain an honest memory from the new Potions master, Horace Slughorn, a elderly man who taught Defense Against the Dark Arts at Hogwarts when Riddle was a student, and who previously provided what was obviously an altered memory.
Riddle has stayed behind after a gathering to question Slughorn alone. He wants to know about the making and use of a Horcrux, an object in which a person has concealed part of their soul:
I don't quite understand how that works, though, sir," said Riddle. His voice was carefully controlled, but Harry could sense his excitement.
"Well, you split your soul, you see," said Slughorn, "and hide part of it in an object outside the body. Then, even if one's body is attacked or destroyed, one cannot die, for part of the soul remains earthbound and undamaged. But of course, existence in such a form....few would want it, Tom, very few. Death would be preferable."
But Riddle's hunger was now apparent; his expression was greedy, he could no longer hide his longing. "How do you split your soul?"
"Well," said Slughorn uncomfortably, "you must understand that the soul is supposed to remain intact and whole. Splitting it is an act of violation. It is against nature."
An act of violation. Against nature. I'm not sure you get much closer to an orthodox account of The Fall without actually quoting from the Philokalia. The language, and the idea, is right out of the Eastern Fathers of the Church. But there's more.
"But how do you do it?"
"By an act of evil -- the supreme act of evil. By committing murder. Killing rips the soul apart. The wizard intent upon creating a Horcrux would use the damage to his advantage: He would encase the torn portion --"
"Encase? But how--?"
"There is a spell, do not ask me, I don't know!" said Slughorn, shaking his head like an old elephant bothered by mosquitoes. "Do I look was though I have tried it -- do I look like a killer?"
Of course, later, the epicurian, comfort-loving Slughorn realizes that through his own careless attitude -- even telling Riddle that it's natural to feel some curiosity about these things....Wizards of a certain caliber have always been drawn to that aspect of magic.... -- he has contributed to the ascent of horrific evil in his world. His response? Instead of doing what he can to rectify his error, to assist those who are braver and more energetic than he, he succumbs to fear and shame and attempts to hide what he has done. The circumstances under which he relents lead me to another conclusion: in certain circumstances, magic in Harry Potter is a symbol of grace. But I think I'll leave that one for another post.