When you ask a question on main, you are greeted with the words What's your christianity question? be specific. I'd say that both Christianity and be should have initial capital letters.
"What's" isn't capitalized either, actually. Also, I looked on MetaSO (twice!) and there is oddly not a single question about this. You could raise it there if you want, but I find it telling that no one else has noticed and posted a Meta question about it. Personally, I noticed, but I didn't care about it as the lack of capitalization actually kinda emphasizes its non-obtrusive appearance.