I currently work in an emergency call center in Texas, we answer 9-11 calls and non emergency calls for police/fire. My religion states that I can not work from sundown on Friday night to sundown on Saturday night.(i know confusing). I asked for my days off to be changed *in writing* so that I can be off of work on Friday/Saturday so that I can keep my religious beliefs and also attend church services on Saturday. My operations manager stated that management did not make scheduling adjustments to accomodate religious beliefs, even though the department I work in runs 24/7.
I even offered to just have Saturday off so I could atleast attend church services and have another day off later in the week even if it was on a different shift. Attending church is very important to be and I would like to know if the management in my department can deny changing my schedule.

I work for a major city in Texas. Any help would be GREATLY appreciated.

An employer is required to make accomodations for sincerely held religious beliefs WHEN IT IS POSSIBLE. There's not enough information in your post for us to determine whether it is possible or not.

If there is an HR department, I would advise you to talk with them and see if there is an accomodation they can come up with. If they refuse, you can always contact the EEOC and get an opinion.