For years, the city of Boston has rotated dozens of flags on a pole outside City Hall to honor veterans, paramedics, sports teams, and LGBTQ pride as part of an effort to promote diversity and civic engagement.

However, when a group applied to hoist a “Christian flag” up the 83-foot pole in 2017, city officials said it would not fly. They claimed that the blue-and-white flag with a red Latin cross in one corner would violate the long-held principle of separation of church and state.

Now, the Supreme Court will hear arguments in that case on Tuesday, which centers on a fundamental First Amendment issue: When a third-party group’s flag flies on a government flagpole, who is sending a message? Which comes first, the group or the government?

Despite a victory last year at the United States Court of Appeals for the First Circuit, even supporters of Boston’s position acknowledge that the city will likely be on the defensive. The high court’s conservative majority frequently looks askance at government regulation of religion, and some justices have expressed concern about “disfavored” religious rights.

Camp Constitution, the group challenging the city, has gained some notable allies, including the Biden administration, which claims that when the federal government allows protests on the National Mall or allows people to submit designs for special U.S. Postal Service stamps to commemorate community events, it faces similar issues.

Staver and his supporters argue that Boston’s flagpole is a public forum, a key concept in First Amendment law that courts use to determine when the government can regulate speech on public property. In a public forum, the government cannot limit speech based on the speaker’s point of view. In practice, this means that if the city allows one group to speak in that space, say an LGBTQ rights group, it cannot prevent a religious group opposing those rights from speaking.

Boston claims that the flag flying above City Hall is a form of government speech, not a public forum for First Amendment purposes, and that city residents perceive the flag as having the city’s approval. Any other approach, according to the city, produces absurd results: A week after raising a flag for the Red Sox, Boston may be forced to fly one for the New York Yankees.

Instead, Boston officials claim that they choose which messages to endorse, and that nearly all of the third-party flags they choose commemorate a day of observance, such as Juneteenth, which commemorates the Emancipation Proclamation, or St. Patrick’s Day. Taken together, they claim, the messages conveyed by the flags are those of the city.

If Boston prevails on that point, the issue of religion – religious freedom and opposing religious organizations – may be sidelined. And this would almost certainly increase the city’s chances of success. The Supreme Court has ruled in favor of religious freedom claims in a variety of contexts, including the COVID-19 pandemic, ranging from churches and synagogues successfully challenging coronavirus restrictions to religious entities successfully opposing requirements that they provide health insurance coverage for contraception.

The court ruled in 2019 that a massive Latin cross on government property outside of Washington, D.C., did not need to be relocated in the name of church-state separation. In 2014, the Supreme Court upheld a centuries-old tradition of opening government meetings with prayers, even if those prayers are predominantly Christian.

On the other hand, a 5-4 ruling in 2015 held that specialty license plates promoting everything from “Choose Life” to “Conserve Water” could prohibit images such as the Confederate flag because license plates are government speech. The decision drew a vehement dissent from Associate Justice Samuel Alito, who was joined by Chief Justice John Roberts and two other justices who have since retired.

The guest flag program in Boston appears to be relatively uncommon. According to a survey conducted by the International Municipal Lawyers Association, more than seven out of ten cities do not fly third-party flags at all.

Even if Boston loses, the Camp Constitution flag is unlikely to ever fly over City Hall. Experts believe that the city, and others with similar programs, will simply stop raising anyone’s flag. Boston has already suspended its guest flag program until the Supreme Court issues its decision, which is expected later this year.