Belief in the resurrection of Jesus Christ is considered to be existential because that event is unprovable and unrepeatable and unverifiable not only now, but across all of human existence. The only evidence is eye-witness testimony. Any evidence can be faked. Relics often are fake. Stories can be fabricated. It happens all the time in court. People want their lies to be accepted as the truth.
But of course that historical event is also existential in nature because of what it implies, uh, religiously. Namely that someone claiming to be God proved it by rising from the dead.
I went to the National Museum of American History in DC when they had the special exhibition on Abraham Lincoln’s assassination. There was presented all kinds of evidence and testimony. This was his tuxedo. This was his top hat. This is a manuscript describing the events that day at the theater. This was the last tea cup he drank from (saved by a White House servant from off a windowsill — creepy by the way!!).
People come to the museum and view this evidence, and I bet most people would never even consider that it could all be a lie, a conspiracy. They were taught by their parents and schools to believe the story. They trust that the Smithsonian employees are doing their jobs, the same jobs that we could do, using the same methods of museum conservation and preservation that we would all be taught and have agreed are worthy of our trust and confidence.
If for some reason the physical evidence and original manuscripts describing Lincoln’s assassination were to be destroyed through war or other calamity, would the historical event be any less true? It is conceivable that someone with a weird agenda, to deny that the event happened, could plot and carry out this sort of scrubbing of history.
Let’s go a bit darker, because I also visited the Holocaust Museum while in DC. Those events were exceptional, seemingly impossible. As time increases, eye-witnesses die, and deniers try to destroy the evidence, the holocaust will also seem more and more surreal, more outside of the capability of human beings, more existential.
Why are there deniers? Because they can’t handle the truth. They want to believe the lie. No amount of preserved or newly discovered evidence, testimony, non-believer documentation, photography, or time travel will ever convince them otherwise.