The victim's belief in possession is what helped cause it... so in that same way the... belief in the power of exorcism can make it disappear.
12 seconds sound clip from the The Exorcist movie soundboard.
You can hear this line at 01:10:08 in the Blu-ray version of the movie.
Quote context
[...]
- Have you ever heard of exorcism?
- But... it, uh, has worked, in fact... although not for the reasons they think, of course.
- It's purely force of suggestion.
- You're telling me that I should take my daughter to a witch doctor. Is that it?
- Karl? Did you put this in Regan's bedroom?
- Karl, if you put this in Regan's bedroom, I want you to tell me. Now, did you?
[...]