Courts have long acknowledged an exception in the law that keeps “ministers” of a faith from bringing employment discrimination claims against their religious employers, capped by a unanimous Supreme Court decision eight years ago.

The idea is that the Constitution forbids the government from becoming entangled in the internal workings and personnel decisions of a religious institution.

But on Monday, the justices will return to the subject with a poignant test: a teacher who lost her job at a Catholic school in California after revealing she had breast cancer.

Kristen Biel won a preliminary battle in her lawsuit alleging the school violated the Americans With Disabilities Act.

But last year she lost her battle with the disease. The school has appealed the decision to the Supreme Court, and her husband, Darryl Biel, has taken her place in the lawsuit.

“I promised her I would see this to the end,” Biel said in a telephone interview from Redondo Beach, Calif. “I wasn’t a big fan at first — I just wanted her to concentrate on getting better. But she was passionate about righting a wrong. I promised her at the end that I would see this through, all the way to the Supreme Court.”

Despite winning at the U.S. Court of Appeals for the 9th Circuit, the Biels, and another teacher who is alleging age discrimination against a different school in the Archdiocese of Los Angeles, face formidable odds. The Trump administration opposes them, as does a long list of religious organizations and scholars.

And most importantly, there is the unanimous 2012 Supreme Court decision in Hosanna-Tabor Evangelical Lutheran Church and School v. Equal Employment Opportunity Commission, where the court found a teacher was covered by the “ministerial exception,” and could not sue her school.

“The interest of society in the enforcement of employment discrimination statutes is undoubtedly important,” wrote Chief Justice John G. Roberts Jr. “But so too is the interest of religious groups in choosing who will preach their beliefs, teach their faith, and carry out their mission.”

He added: “The First Amendment has struck the balance for us. The church must be free to choose those who will guide it on its way.”

The cases to be argued Monday call on the court for further guidance about what kinds of employees should be regarded as ministers and what role courts should play, if any, in looking behind the religious organizations’ decisions on which employees are covered by the exception.

“Holding the ministerial exception can be triggered simply by showing that an employee performs ‘important religious functions’ would turn the exception inside out,” Jennifer A. Lipski and Jeffrey L. Fisher wrote in a brief for Biel and the other teacher, Agnes Morrissey-Berru. “Countless employees of religious institutions — not just lay teachers, but also nurses in hospitals, counselors in summer camps, cooks and administrators in social services centers, and other categories of workers — perform duties that their employers sincerely consider important to their religious missions.”

Eric Rassbach, senior counsel for the Becket Fund for Religious Liberty, which is representing the schools, said the 9th Circuit ruling that allowed the teachers to proceed with their suits is out of line with the Supreme Court’s decision and those of other appeals courts around the country.

The teachers and their allies made the same arguments before the 2012 Supreme Court decision, he said. “This [exception] has been around for a very long time, and the sky hasn’t fallen in the lower courts. I don’t see why it would fall now.”

The justices in 2012 said there was no rigid test for deciding which employees fell into the ministerial category. Courts should look at the employee’s title, whether the organization considered the employee a minister, whether the employee considered herself a minister and whether the employee’s duties included “important religious functions.”

And Justices Samuel A. Alito Jr. and Elena Kagan added a broadening concurrence, pointing out that some faiths don’t even use the term minister.

If Hosanna-Tabor was a major decision in the world of religious organizations and employment discrimination lawyers, it was not elsewhere.

“My wife was hired as a fifth-grade teacher. She wasn’t hired as a minister,” Biel said. “The first time we heard the word ‘minister’ was after the lawsuit was filed. There’s no one I’ve ever told this to who wasn’t shocked.”

Kristen Biel was a dancer and aspiring actress when the couple met, and she became a stay-at-home mom after they married and had two children. When the children grew older, she wondered what she should do outside the home.

“Everyone told her she’d be a wonderful elementary school teacher,” Darryl Biel said, so Kristen went back to college and got a teaching certificate.

Their children had attended parochial schools for most of their education, Biel said, and Kristen converted to Catholicism with the hope it would improve her chances of getting a job in a Catholic school.

Both sides acknowledged that her work as a fifth-grade teacher included teaching religion, joining students in prayer and taking her class to Mass once a month. The Biels contend her training consisted mainly of following a workbook; the school says religion is an important component in everything its teachers convey to students.

Kristen Biel was diagnosed with an aggressive form of breast cancer during Easter break in 2014. She told the head of school when she returned that she would need time off for treatment. Later, she was informed her annual contract would not be renewed.

Darryl Biel said the principal told his wife that it would be unfair for students to have a different teacher during the times she would miss, and that her treatment of chemotherapy and a double mastectomy might be “a little traumatizing to the kids to see what kind of transformation might take place to Kristen.”

“I remember this like it was yesterday, my wife coming in the house after having that discussion with Sister Mary Margaret,” Darryl Biel said. “She was devastated. Crying inconsolably. And when I finally got her to calm down, that’s when she told me everything Sister Mary Margaret said.”

The school has a different version of events: Kristen Biel had been warned before the diagnosis that she was not performing well, Rassbach said. Her classroom was chaotic and the children were not thriving. Parents had complained, he said.

The competing versions would be weighed in the legal process, if the Biels’s lawsuit is allowed to proceed. A district judge dismissed the claim, agreeing with the school that Biel was covered by the ministerial exception. A divided panel of the 9th Circuit decision revived it, saying only one of the considerations the Supreme Court set out in Hosanna-Tabor favored the school.

Civil rights groups, including the American Civil Liberties Union, urged the Supreme Court to be cautious in the case.

“The ministerial exception serves important purposes in safeguarding religious institutions’ autonomy with respect to governance and leadership. But it comes at significant cost,” the groups told the court in a brief. “It confers on religious institutions the extraordinary power to discriminate against ministerial employees on any basis whatsoever, including race, disability, sex, and age.”

A court’s examination requires nuance, they said, and the two cases before the Supreme Court demonstrated that. In the view of the groups, Biel should not be considered a minister. But they say Morrissey-Berru should, because her position required her to be Catholic and certified in religious education.

A coalition of leading scholars who study religion and the law told the Supreme Court that the 9th Circuit got it wrong. Unless the rulings are overturned, “they will invite judicial intrusion into religious affairs and create confusion regarding the autonomy of religious bodies to choose those who perform significant religious functions.”

And the Trump administration said the court should make it clear that it is the religious organization that has the final word on who is a minister, and who is not.

“In close cases, facts that demonstrate a religious organization sincerely regards its employee as performing such important religious functions should be dispositive,” Solicitor General Noel Francisco told the court in a brief.

The cases are Our Lady of Guadalupe v. Morrissey-Berru and St. James School v. Biel.