can i be fired for requesting sundays off for church

You asked too general a question for a definite answer. You may request that you not be scheduled to work on Sunday so you may attend services. However, the employer is not required to accommodate your request if by doing so, he would disaccommodate other employees.

If your religion actually prohibits you from working on Sundays (not just that you want to attend services), and your job requires working Sundays, then you are not able to perform the essential functions of the job and you may be discharged for that; not because you asked for time off to attend church.

As you might imagine, being as Sunday is the sabbath for many religions, it would not be unusual for several employees to never want to work Sundays and the employer is not likely to be able to accommodate everyone.