> It makes for an excellent litmus test to ask, sometimes literally out loud, whether someone who advocates for the legislation of a specific religion's rules would be OK with being forced to comply with a different religion's rules and tenets.
This overlooks the difference between majority religions and minority religions. If the majority want to legislate a rule, it shouldn’t matter what is the rationale behind that rule. Everyone relies on moral frameworks based on non-falsifiable axioms to formulate political opinions.
In the German state of Bavaria, all public buildings are required to display Christian crosses: https://www.bbc.com/news/world-europe-43892329. This is a reasonable exercise of democratic will in a Christian-majority place.
A better question is to ask the same person how they feel about legislation motivated by religious principles in a country where the majority follow that religion. I expect you’d find that most American Christians don’t care if India wants to legislate based on Hindu principles.
> A secondary litmus test is whether a person can even understand the reasoning behind the idea of the separation of church and state
A better litmus test is whether someone can identify where in the Constitution it says anything about “separation of church and state.”