The U.S. Supreme Court heard oral arguments in a case concerning an evangelical Christian and former U.S. Postal Service worker, who was denied an accommodation to observe his Sunday sabbath by not taking Sunday shifts.

Federal law prohibits employers from firing employees seeking religious accommodations unless the employer can show that those accommodations cannot be “reasonably” met without “undue hardship.” In a 1977 decision in Trans World Airlines v. Hardison, the high court found that the “undue hardship” standard is met even at a minimal cost.

Gerald Groff said he sought employment at the post office since it did not deliver mail on Sundays, however, that practice changed during his employment there, leading him to seek an accommodation. However, the USPS refused.

The 3rd U.S. Circuit Court of Appeals ruled in favor of USPS, arguing the post office would face an “undue hardship” by accommodating Groff’s request to excuse him from Sunday shifts. But the U.S. Supreme Court agreed earlier this year to take up the case.