A chiasm is a literary structure where vocabulary of the first section of a passage is repeated in reverse order in the second. The center of the chiasm is typically the climax of the passage. In John 10:17-18 the center and climax is Jesus saying that no one will take his life from him but that he will lay it down on his own initiative (John 10:18a-b). This is also the center of the chiasm of the whole gospel of John.