explain the meaning behind karras self sacrifice death and salvation in the end of the exorcist

This paper needs to be 3 pages long and follow MLA standards. It must have integrated quotes from the novel The Exorcist by William Peter Blatty. A copy of this novel is needed in order to write this paper. The paper also needs to be supported by a secondary source, and for the case of this paper a source written about the film is fine since the author of the novel wrote the screenplay for the film. The secondary source cannot be film criticism, it has to be a source of academic value so anything from a journal article or a book will be fine.