You’re probably aware of the idea of Limbo. No, not the party trick. The concept of a place of eternal waiting. Not very good, not very bad. Just… not much of anything, forever. Here’s the technical religious definition:
Limbo, which comes from the Latin word meaning “border” or “edge,” was considered by medieval theologians to be a state or place reserved for the unbaptized dead, including good people who lived before the coming of Christ. (source)
Limbo has never been part of any official Catholic doctrine, although it’s been taught to Catholics for centuries. I first learned about it from Dante, who visits Limbo (located outside the gates of hell) in the Inferno. Dante places well-known, respected historical pagans like Socrates and Plato in Limbo, but argues that righteous Biblical figures like Abraham were plucked from their eternal condition by Christ when he descended into hell following the crucifixion.
The existence of Limbo has been pretty thoroughly nixed in recent years; Catholics from the pope downward basically agreed that Limbo didn’t exactly mesh with the idea of a loving God, since the aforementioned unbaptized dead included the souls of children with no personal sin. Although it wasn’t entirely ruled out, a Church document released a few years ago points out that “People find it increasingly difficult to accept that God is just and merciful if he excludes infants, who have no personal sins, from eternal happiness, whether they are Christian or non-Christian.” (source)
Regardless of whether Limbo exists or not, or who is or isn’t in it, the concept has meshed itself into pop-culture enough that even a non-religious person will know what you mean if you say that something is “in limbo”.
Inception’s worldbuilding focused heavily on the concept of Limbo, actually including a final dream-level called Limbo as a major plot point. This realm is a universal location into which any dreamer who goes too deep or dies in the dream gets funneled. Once there, time moves infinitely faster than it does in the waking world. It’s hard to keep hold of yourself, and without outside interference, you can grow old and die without ever waking up from the dream. This isn’t a remotely religious limbo, but hey—they could have called it anything. Calling it Limbo was an intentional and evocative choice, because it carries cultural significance.
And that’s just the most specific example I can think of. There are plenty of value-neutral post-death waiting places in pop culture. There is the waiting room (and really most of the afterlife) in Beetlejuice. In Marvel comics, Limbo is a plane outside time, ruled over by a future version of Kang the Conqueror called Immortus. Harry’s post-death King’s Cross in Harry Potter has elements of Limbo, as does the Void space in Doctor Who.
Limbo isn’t an official Catholic doctrine, and it never really was. But it’s fascinating to see how far reaching a religious concept can be—especially to the point where it has basically lost its religious connotations and is understood by pretty much anyone you talk to, regardless of affiliation or belief.