Religion at Work

I was employeed by Kroger 776 in Marietta ohio over 8 years ago. When I was hired, I told them I would work 16 hrs a day if they wanted, Monday through Saturday. I told them that Sunday, I wanted off because I feel its the Sabbath Day, and God Created the world in 6 days and the 7th he rested. So therefore, I don't believe any place should be open on Sunday & I will not work it. They approved my Sunday off for over 8 years, And now they want to try to say its a part of my work Schedule & if I deny the shift.. then I can't bump anybody else for hours which means take hours from anybody with less senority due to being in a union. Can they really legally discriminate against me and my religious beliefs like this ? They said they will deny my bumps if I give away my Sunday Scheduled shifts... if not, I would like this to be handled & reimburrsed for my discimination & harrassment. Please contact me soon with an answer. Thanks, and God Bless you all, Justin Hill