Everything sad will come untrue, but not yet
The line comes from that master of the storytelling art, JRR Tolkien, in The Return of the King:
“Gandalf! I thought you were dead! But then I thought I was dead myself. Is everything sad going to come untrue? What's happened to the world?"
A great Shadow has departed," said Gandalf, and then he laughed and the sound was like music, or like water in a parched land; and as he listened the thought came to Sam that he had not heard laughter, the pure sound of merriment, for days upon days without count.”
And yet this idea, or this refrain, "everything sad is going to come untrue", has become popularised in a certain segment of Christian discourse to encompass eschatological hope. I think that goes back primarily to Tim Keller, who uses it as a reference point in his sermon on 9/11, and then throughout some of his other writings. For Keller, it is a kind of leitmotif, and from that it has entered broader usage.
That, I think, is the danger of catchphrases. They lose their meaning, and so lose some of their force, the more they get trotted out. Then the danger is they become platitudes. A platitude is marked by being trite, and triteness (etymologically) occurs when something that was meaningful has been worn away by usage.
Twice in recent memory I can recall people saying to me, observing my sorrow, a version of "well, don't worry, it will all be set right in the world to come". Let me say, that is true, but it is not a comforting truth in the moment of pain. It is salt on a wound, not a salve.
Why is it salt though? Because in offering a platitude about eschatological hope, we try to treat pain by diminishing the reality of that pain. It's the counselling equivalent of "you don't need treatment, your wound isn't that bad". True medicine doesn't downplay the severity of the wound, it treats the wound as it is.
I think "everything sad is going to come untrue" is profoundly true, but easily misapplied. It sounds too much like that we hope that all the sad things of this world are going to be rewound and played forward again and turn out differently. That history will be written over, and the evil, sin, sickness, sorrowing will be blotted out and forgotten and made to be "it never happened" away.
That is not the Christian hope, nor is it what Gandalf is saying! Sam Gamgee feared that Gandalf was dead. And that he was dead. Gandalf, in a very Tolkienesque way, was dead. But he came alive again. That wasn't retconning Gandalf's backstory, it was death followed by resurrection. Resurrection makes death untrue, it doesn't make the death that he died a non-real past event.
So too when we look at the paradigmatic event of Christian hope - the resurrection. There Jesus' being dead becomes untrue. Untrue in this sense is a ontological unravelling of evil. So it's not that Jesus stopped "having a past that involved his dying on the cross", it's that the present reality of being dead is made untrue, because he is alive.
I don't know what the eschatological return of Jesus in triumph looks like in practical terms. I do know that it involves God wiping away every tear from his people's eyes, death no longer existing, mourning and crying and pain having ceased to exist (Apoc 21.4). That is not a diminishment of the pain of the world that has passed, but a healing and a renewal for the world to come.
That is a true comfort, one that lets us sit in the sorrow and neither distract ourselves, nor diminish and demean the pain, nor offer easy remedies, but offer an ultimate hope that has the space to mourn with those who mourn.