The answer to the final question about the "outcome" of your challenge need not be a success story.
Sometimes in retrospection, we discover that the cost of an action was perhaps too great.
Introspection and honesty are key with this prompt. The "belief or idea" you explore could be your own, someone else's, or that of a group.
The best essays will be honest as they explore the difficulty of working against the status quo or a firmly held belief.
However you approach this prompt, your essay needs to reveal one of your core personal values.
If the belief you challenged doesn't give the admissions folks a window into your personality, then you haven't succeeded with this prompt.
The essay prompts are designed to encourage reflection and introspection.
If your essay doesn't include some self-analysis, you haven't fully succeeded in responding to the prompt.
Some students have a background, identity, interest, or talent that is so meaningful they believe their application would be incomplete without it. The prompt gives you a lot of latitude for answering the question since you can write a story about your "background, identity, interest, or talent." Your "background" can be a broad environmental factor that contributed to your development such as growing up in a military family, living in an interesting place, or dealing with an unusual family situation.
If this sounds like you, then please share your story. You could write about an event or series of events that had a profound impact on your identity.