Final answer:
The provided text does not explicitly answer why Christine changes her mind about destroying the letter, but it suggests that the 'incredible things' may refer to realizations that lead to a reconsideration of actions, as seen in situations where characters deal with the unseen consequences of their decisions.
Step-by-step explanation:
The passage you've provided does not give direct insights into Christine's change of heart regarding the destruction of the letter. However, we can infer from similar literary contexts and the provided excerpts that moments of realization or confrontation with the consequences of one's actions can cause such a change. The 'incredible things' could refer to unexpected outcomes or realizations that dramatically shift a character's perspective, leading them to reconsider their initial intentions. In a situation like Krogstad and Mrs. Linde's conversation, we see that even when wrongdoing has been committed, a second chance or redemption might seem possible, suggesting why someone might relent from a harsh decision.
It's also implied through various texts that characters struggle with the unseen impacts of their actions. A decision that seems straightforward can haunt a location or person due to its ethical or moral implications. For example, the text referencing Lucy Dare hints at a lasting spiritual impression caused by her decision, which can resonate with the idea of second-guessing one's drastic actions like destroying a letter.