This is a big argument about what is "real," I think.
"An abstraction" could legitimately be called "all in your head" ... but the Pope specifically denies that Heaven or Hell are mere abstractions. "A place in the clouds" would imply that one could theoretically get into a sufficiently powerful spaceship and fly there ... a spectacularly childish (not childlike) notion of Heaven or Hell, which again the Pope rejects.
In order to get to Heaven or Hell, one must die ... the Pope asserts that they are indeed real, but of an order of reality different from space and time. If one rejects the possibility of a reality different from space and time, then the Pope's words will make no sense. Neither will the Bible make any sense.