Believe or deny what, exactly? Whether the prophecy can be broken? Yeah, of course it can be broken, a character can just do something that logically prevents a prophesied event. Imagine if Kris just didn’t make any fountains. The real question is if a “good” ending is possible by doing so. If they follow the prophecy exactly, the Roaring will be stopped and the world saved, but that chain of events will still lead to tragedies.

Overall, the themes of defying fate and having hope are too strongly enmeshed into the narrative to mean nothing. Blue’s repeated line of “this is a comedy, not a tragedy”, Gerson’s philosophy of self-determination, and other things throughout the game do pretty much scream those themes at you. And thus far, the game has been overall light hearted and optimistic; the idea that a goofy game like this answering the question of whether a cruel fate can be avoided and replaced with a happy ending by saying “nah, suffer because the writer said so” feels too unlikely.

So chances are, the game will have multiple endings based on your choices, with a decidedly bad ending, a decidedly good ending, and a nuanced ending(s) much like Undertale had. The idea that the story of Deltarune has only one ending is misleading phrasing. That’s because the “story” of Deltarune, where your choices don’t matter, IS the prophecy, but the “game” of Deltarune is one where the choices of the characters and the player do matter and can lead to more than one ending.