Supreme Court rules Boston violated First Amendment rights by refusing to fly Christian flag at City Hall Plaza

City Hall Plaza in Boston.

A unanimous US Supreme Court ruled Monday that the City of Boston violated the First Amendment rights of a Christian group by refusing to fly a flag bearing a cross outside City Hall in 2017.

In a ruling written by retiring Justice Stephen Breyer, the high court said that Boston was wrong to deny Camp Constitution, run by West Roxbury resident Harold Shurtleff, a permit to raise a white banner with a red Christian cross in connection with Constitution Day on Sept. 17, the date the US Constitution was signed in Philadelphia in 1787.

“We conclude that, on balance, Boston did not make the raising and flying of private groups’ flags a form of government speech,’’ Breyer wrote. “That means, in turn, that Boston’s refusal to let Shurtleff and Camp Constitution raise their flag based on its religious viewpoint” abridged their freedom of speech.

It was the only time the city refused to fly a flag, having approved 284 requests from various countries, causes, businesses, and organizations between 2005 and 2017, the court said. The American Civil Liberties Union and the Biden administration had filed briefs supporting Camp Constitution in the case.

“While in most cases displaying a religious flag on government property would violate the Establishment Clause, where a city chooses to turn its flagpole into a public forum for all comers, it can’t turn away Camp Constitution’s flag because it’s religious,” said David Cole, national legal director for the ACLU.

The high court ruled that if the city had expressly endorsed the message printed on the flag, it would have qualified as government speech, meaning the city could accept or deny requests as it chose.

“The First Amendment’s Free Speech Clause does not prevent the government from declining to express a view,” Breyer wrote. “That must be true for government to work. Boston could not easily congratulate the Red Sox on a victory were the city powerless to decline to simultaneously transmit the views of disappointed Yankees fans.”

But the city blocked Shurtleff for no other reason than the religious message his flag carried, violating his free speech rights, Breyer wrote.

“While the historical practice of flag-flying at government buildings favors Boston, the city’s lack of meaningful involvement in the selection of flags or the crafting of their messages leads us to classify the flag raisings as private, not government, speech — though nothing prevents Boston from changing its policies going forward,” Breyer wrote.

Boston Mayor Michelle Wu will now develop policies for the flag displays, her office said. City Hall typically flies three flags — for the US, the commonwealth of Massachusetts, and the city of Boston. The city’s flag is the one replaced during ceremonial flag-raising.

”We are carefully reviewing the Court’s decision and its recognition of city governments’ authority to operate similar programs,’' a Wu spokesperson said in a statement. “As we consider next steps, we will ensure that future City of Boston programs are aligned with this decision.”

Shurtleff’s lawyer, Mathew D. Staver of Liberty Counsel, a nonprofit dedicated to advancing religious freedom, said the city’s position was so legally flawed it forged consensus between the court’s conservative and liberal wings.

“Those kinds of decisions when you have unanimous decision are very rare,” he said. “There is a clear message from the Supreme Court that Boston’s censorship was unconstitutional . that Boston crossed the line. They had no defense.”

Staver said that Camp Constitution would apply to have the same flag, or one with a similar Christian theme, flown from a city flagpole, most likely on Constitution Day in September. Staver said the court’s ruling means formal guidelines the city enacted in 2018 cannot be enforced.

“They put into writing what they had been doing for the previous 12 years. And what they had been doing for the previous 12 years was rejected by the Supreme Court,” he said.

But Americans United for Separation of Church and State, an advocacy group, said the ruling could be abused by governments who allow only some religions a platform.

“If a government permits this access to the majority religion, others are equally entitled to display their beliefs or non-belief, too,” Rachel Laser, the group’s president, said in a statement. “This decision is a reminder that groups representing religious minorities and nontheists must be given equal access to these kinds of public forums to display their own symbols.”

Roy Gutterman, director of the Tully Center for Free Speech at Syracuse University, said the decision was narrowly focused.

“The main issue here is what would constitute government speech,” he said. “And I think the court here acknowledges that people can tell the difference between a statement that the government is making or endorsing and having a venue that anybody can opt to use.”

If the city wants to use the flagpole as a public venue, it must be available to all, he said.

“A government official made a good faith decision here, that because of the [religious] nature of this flag that it might be construed as the state or the city endorsing a political or a particular religious viewpoint,” he said.

In a concurring opinion, Justice Samuel A. Alito Jr. agreed with Breyer’s conclusion, if not its logical basis.

“A program with this design cannot possibly constitute government speech,” he wrote. “The flags flown reflected a dizzying and contradictory array of perspectives that cannot be understood to express the message of a single speaker.”

John R. Ellement can be reached at john.ellement@globe.com. Follow him @JREbosglobe. Gal Tziperman Lotan is a former Globe staff member.