Why does King want his readers to put his book down if they are unable to accept that writing is magic?

King believed that anyone who does not believe that writing contains elements of magic, and aspects that are highly unexplainable, will be unable to benefit from his book, On Writing, because they will be unable to accept King's fundamental beliefs about writing. Since the reader (or writing student) would be unable to accept King's fundamental beliefs, there would be no purpose in continuing to read his book. Therefore, they should stop reading the book.