All you need to say is that you are exempt for religious reasons. They can pry only as far as it affects your accommodation. In an ideal world the conversation would go something like this: You: "I'm exempt for religious reasons." Them: "Do you mind wearing a mask?" You: "I'm exempt from that, too. If I get sick I'll stay home." Them: "Ok. You can come in on Monday." If they try to ask questions like "Do you believe vaccines work?" or "Have you ever taken a vaccine?" you can tell them that you'd prefer not to discuss your personal or religious beliefs but that you're happy to discuss accommodation.