I think there's a large cultural bias at play here. Different nations have different relationships to religion. As a french person, the decision to mark religious content as NSFW seems totally normal to me, but I also know that french people are (often too) fierce atheists.

I also understand things are different in many places, but I think the argument is too heated right now, maybe everyone needs to take a step back and think in a more "international" way?

Someone in the linked thread suggested a new tag altogether for religious content, that might be a sound decision.