U.S. Supreme Court justices grappled Tuesday with when employers may refuse employees’ workplace religious accommodation requests in a case concerning an Evangelical Christian mail carrier who didn’t want to deliver packages on Sundays.

At issue is the high court’s nearly 50-year-old precedential Trans World Airlines v. Hardison decision which states that employers don’t need to provide religious accommodations to employees if it would create an “undue hardship.” Under the ruling, “undue hardship” means an accommodation would impose more than a minimal burden on the workplace.