So the religion I follow does not permit working on our sabbath, which falls on Saturday. I specifically mentioned in my interview that there's no way I'd be able to work on Saturdays, but I didn't say that it was for religious reasons - I've always thought that it's best to not bring religion into the workplace, and up until now it hasn't been a problem.
Now my manager has decided that we'll all work next Saturday to complete some project. I've told them that I can't do it, and they've said that it will be required of us and cannot be missed for any reason. I'd much sooner lose this job than go against my religion, so that's what it's looking like will happen. I don't mind my job though, and it would be nice to not have this happen, so I'm hoping we can work this out. I think my manager has taken "I'm not available on Saturday" and "I won't be able to work on Saturday" to mean that I just don't want to, or I have some other more flexible plans. Would it be a good last resort to mention that I'm not able to do this for religious reasons, or should I just turn in my notice?
Note: I'm not interested in getting into a debate about my religion, or sharing more specifics about it, both with my manager and with this online group. I'll just leave both if it comes to that.