I know that where I work and go to college (same place), declaring that I'm not Christian but an atheist (or budhist, or fill in the blank, whatever) would accomplish two things immediately:
1) My boss (head of the department and rabid evangelical Christian) would immediately see me as less desirable as an employee. Would I get fired? Probably not, but it would hurt my employment opportunities and future reccomendations from her. Is that legal? Hell no! But that wouldn't matter because I couldn't prove anything.
2) My instructors (many of which are vocal about their Christian beliefs) would immediatly see me in a lesser light and that would affect my grades on work that is graded on a subjective nature. Is that fair? Legal? Again, hell no, but I wouldn't be able to prove it. I have one instructor who I have had for several different classes. He starts out every first day of a new semester with a HUGE Power Point presentation about himself. It is mostly about his Christianity. He has gone so far as to hand out invitations to bible studies at his home. We don't dare complain because class size is limited to a maximum of 16 people. Gee, wouldn't be too hard to figure out who squeeled.
So, I try to keep my mouth shut about my beliefs (at least offline - here, that's different).