Not much other than the question. Can a workplace for example ask if you are Catholic, Muslim, Jewish, Jehovah's Witness via survey... and then sub-questions based on your answers? I want cases and law not yes/no with "I have seen this".
It's sort of a toss-up. EEOC says that "Questions about an applicant's religious affiliation or beliefs (unless the religion is a bona fide occupational qualification (BFOQ)), are generally viewed as non job-related and problematic under federal law". They also state that (excluding positions with religious exemptions, like hiring a Catholic priest) "Other employers should avoid questions about an applicant's religious affiliation, such as place of worship, days of worship, and religious holidays and should not ask for references from religious leaders, e.g., minister, rabbi, priest, imam, or pastor". This reflects the standards by which they will make a finding of forbidden religious discrimination.
The actual statutory law, 42 USC 2000e-2 does not prohibit asking questions about religion, instead it says
(a) It shall be an unlawful employment practice for an employer— (1) to fail or refuse to hire or to discharge any individual, or otherwise to discriminate against any individual with respect to his compensation, terms, conditions, or privileges of employment, because of such individual’s race, color, religion, sex, or national origin; or (2) to limit, segregate, or classify his employees or applicants for employment in any way which would deprive or tend to deprive any individual of employment opportunities or otherwise adversely affect his status as an employee, because of such individual’s race, color, religion, sex, or national origin.
29 CFR Part 38 Subpart A does not specifically prohibit asking questions. Although many labor-law web sites and HR advice firms state that asking questions is illegal, DoL and EEOC do not specifically declare categorical illegality. The fact of asking some question might constitute partial evidence for a finding of religious discrimination. So no court has held that it is flatly illegal for an employer to ask a question about religion.
The case of EEOC v. Abercrombie & Fitch, which did find discrimination, gives you the required elements for a finding of religious discrimination. There is simply no "don't ask" cause of action. Asking might be evidence to support a claim of disparate treatment based on religion. The rule is: "An employer may not
make an applicant’s religious practice, confirmed or otherwise, a factor in employment decisions".
I have found no cases where EEOC found illegal discrimination based exclusively on the fact of asking questions about religion.