It’s strictly forbidden to have a religious test for office in the United States, but it is reasonable to ask a person if their moral, social, political, religious or economic convictions will interfere with their ability to do their job. After all, it’s not just religion that can cause a problem. People have non-religious convictions that can interfere with their ability to judge impartially.
How far can you go down the road of questioning the influence of a person’s religious beliefs without breaking the prohibition on a religious test for office?
There are some things that would clearly be illegal religious tests, such as rule that a person has to be a Trinitarian, or swear allegiance to the pope, or be baptized in order to hold office.
But it gets tricky the further you get from such obvious examples. What about requiring that a person hold to a religious point of view — say, that all men are created equal? Can we require that from a candidate? It’s an extraordinarily useful dogma that lies at the heart of our system and laws, but … it really is a religious idea. What if someone didn’t hold to it? Would that disqualify him?