"Digory was just turning to go back to the gates when he stopped to have one last look around. He got a terrible shock. He was not alone. There, only a few yards away from him, stood the Witch. She was just throwing away the core of an apple which she had eaten. The juice was darker than you would expect and had made a horrid stain round her mouth. Digory guessed at once that she must have climbed in over the wall. And he began to see that there might be some sense in that last line about getting your heart's desire and getting despair along with it. For the Witch looked stronger and prouder than ever, and even, in a way, triumphant; but her face was deadly white, white as salt." C.S Lewis, The Magician's Nephew, Chapter Thirteen.
Or, in the plain words of Scripture: "Verily, verily, I say unto you, He that entereth not by the door into the sheepfold, but climbeth up some other way, the same is a thief and a robber." John 10:1.