To answer your question Yes if he is trying to keep you from your right to free speach. Or descriminating due to your religious followings, But If they are you are not going to be able to prove it as your boss will simply say he is trying to run his business and that includes you being scheduled to work on Sundays.
I wish it was so easy to prove. It would be nice if employeer's that saw fit to be open on Sundays would let those that want to attend Church go. God knows it only improves moral when I get to go and creates negative energy when I don't.
Be Blessed in your endevaors.

