Generally speaking the courts recognize that it is legitimate for an employer to expect everyone to show up for work when scheduled. As long you work in a job category where all the employees are expected to work on Sunday, for example, then you can generally expected to do so too. However, if you can show that your employer could accommodate your need to attend services on Sunday. without any significant expense , then the employer is obligated to do so. If you can find an employee who is willing to trade shifts with you, or who will simply work the shift because he wants the overtime, then the employer has to let you do so. However, the company does not have to force another worker to take your shift for you.
Flexibility is important for both sides.
If the company refuses to see what it can do to accommodate the employee who seeks an accommodation of his need to attend religious services on Saturday, then the company will probably lose. If the employee refuses to try to find a substitute worker and tries to put that obligation on the company, then the employee will probably lose. Congress has made a couple of attempts at laying down the law in this area. It has done so in an attempt to promote freedom of religion. The first attempt was much stronger in it's protection of the religiously observant employee.