Asking about your religion on the census is against the law in the US:
no person shall be compelled to disclose information relative to his religious beliefs or to membership in a religious body.
https://www.congress.gov/94/statute/STATUTE-90/STATUTE-90-Pg...
> compelled
Doesn't that mean they can ask that question with an option for "rather not disclose"?
You can look at the sample form for 2020 census and you will find it is not on there.
Religion is just an example. Don't dwell on it.