I think you are getting some really good advice here.
I would add this perspective: Have you created a master list of all the things that you want your applications to show about you? Some sort of document that reflects the most important things every school needs to know about me (could be 4-5 bullet points, a few sentences, or whatever). If not, I would strongly strongly suggest that.
Now that you have this list or document in front of you – is Model UN the biggest, most important item on that list? If not, it really has no business being the star of the biggest essay.
Is religion your biggest part of that document-about-you? If not, then it has no business being your biggest essay. (I bring this is up to show you how absolutely this should NOT be your essay, to answer your original post, unless truly, religion really is the biggest part about you. It might be. But it sounds like you were distracted by the question, not digging deep into your soul here.)
I guess what I am suggesting is: get clear on who you are, and what you need to share to communicate that clearly, and write it down on a document. And then: Stay focussed! Don’t get distracted by these super interesting open-ended questions! Use them to tell your story. What is your story? It’s all right there in your document. I’ll say it again because it’s important: Use their questions to guide you into telling YOUR story. (Yes answer the question. But answer it with your story. So you have to do two things at once here: Manage their actual question while also revealing your true self.)
One other thing to note: Your “list” may not be “activities.” In fact, in my opinion, it’s more compelling when they aren’t just a bunch of activities/ECs/academic subjects. I prefer to think of them as “themes” or “the point or takeaway” of an activity. For example, if you like basketball and skydiving, then maybe the bullet point is you love adrenaline and adventure. Now basketball and skydiving are examples of the “thing” but they themselves aren’t the actual “thing.” Does that make sense? An essay about adrenaline and how you love it (working in basketball and skydiving as the examples) and how that will have you trying new things at BS is more interesting than an essay about basketball.
Also: in case you haven’t then already noticed, using themes has additional advantages: it allows you to work in more than one activity in a way that feels natural. And, it does the hard work for your admissions team to categorize yourself for them. (Rather than forcing THEM to figure out: oh he must love adrenaline rushes because he loves multiple fast-paced sports ). And it makes it easier for them to make the mental leap to picturing you there when you say: So this makes me imagine I would love the rush of being on the water so I want to try crew when I come to your school. (Or whatever. I am making up an example, and admittedly it’s not great.)