The court ruled unanimously to throw out previous rulings against former postal worker Gerald Groff, who wanted to take off on Sundays due to his religious beliefs and ultimately resigned from the U.S. Postal Service and sued the agency.

USPS management forced Groff to work on Sundays when he was scheduled when someone couldn’t be found to replace him, and disciplined him when he did not, which he said violated his federal rights.

While federal law requires accommodations for employees’ religions unless they would impose “undue hardship” on the employer, previous Supreme Court precedent has found employers shouldn’t be required to “bear more than a de minimus cost” to accommodate a worker’s religion—which Groff’s lawsuit challenged, arguing he should have been granted accommodations and the court should change its precedent.

The Supreme Court threw out that “de minimus” standard and clarified when workers’ religions can’t be accommodated, saying workplaces must instead show that the accommodation would “result in substantial increased costs in relation to the conduct of its particular business.”

Employers must “reasonably accommodate” workers’ religions and not just “assess the reasonableness of a particular possible accommodation or accommodation,” the court ruled, meaning an employer couldn’t just “conclude that forcing other employees to work overtime would constitute an undue hardship” in a case like Groff’s.

The court sent Groff’s case back to the lower court, which will now decide based on the new standard if the USPS acted unlawfully by requiring him to work on Sundays.