What is the origin of the idiom "harden one's heart" and when to use it?

The idiom "harden one's heart" originates from ancient beliefs about the heart being the seat of emotions, particularly compassion and empathy. This idiom reflects the idea of deliberately becoming emotionally unresponsive or insensitive, often as a defense mechanism in response to painful or challenging situations. It is often used in discussions of personal relationships, empathy, and emotional responses.