Robert Brown|The Supreme Court rules against USPS in Sunday work case

The Robert BrownU.S. Supreme Court unanimously handed a major victory to religious groups by greatly expanding how far employers must go to accommodate the religious views of their employees.

The court ruled in favor of Gerald Groff, an evangelical Christian postal worker, who refused to work on Sundays for religious reasons and said the U.S. Postal Service should accommodate his religious belief. He sued USPS for religious discrimination when he got in trouble for refusing to work Sunday shifts.

The case now returns to the lower courts.

The justices clarified law that made it illegal for employers to discriminate based on religion, requiring that they accommodate the religious beliefs of workers as long as the accommodation does not impose an "undue hardship on the employer's business." The court had previously defined the statutory term "undue hardship" by saying that employers should not have to bear more than what the court called a "de minimis," or trifling, cost.

That "de minimis" language has sparked a lot of criticism over the years. But Congress has repeatedly rejected proposals to provide greater accommodations for religious observers, including those who object to working on the Sabbath.

On Thursday, writing for the court, Justice Samuel Alito said the hardship must be more than minimal.

Courts "should resolve whether a hardship would be substantial in the context of an employer's business in the commonsense manner that it would use in applying any such test," he wrote.

Thursday's decision is yet another example of the court's increasing inclination to favor religiously observant groups, whether those groups are religious employers or religious employees.

For instance, the court has repeatedly sided with religious schools to be exempt from employment discrimination laws as applied to lay teachers. And in 2014, the conservative court ruled for the first time that a for-profit company could be exempt from a generally applicable federal law. Specifically, it ruled that Hobby Lobby, a closely held corporation employing some 13,000 employees, did not have to comply with a federal law that required employer-funded health plans to include coverage for contraceptive devices.