How can one atone for taking a cross-shaped knife from one's aunt's house without their permission, then breaking it based on another person's instruction due to the presence of the cross on it, all without the aunt's knowledge?

It is forbidden for a Muslim to keep anything in the form of a cross. It is obligatory to remove and efface it, for it was narrated from Aisha (may Allah be pleased with her) that the Prophet (peace and blessings of Allah be upon him) did not leave anything in his house that contained crosses but that he broke it.

The jurists have stipulated that there is no liability upon one who breaks a cross, and that its removal is among the evils for which a Muslim is rewarded, including a discerning child.

However, a Muslim should not hasten to break every cross he sees, especially if it belongs to someone else. The removal of evil is conditioned upon the safety of the outcome and the absence of a greater harm. It is limited to advising with the tongue. For the removal of these things is not for individuals, but rather for the rulers.