A Christian group is threatening legal action if the Walsh administration doesn’t allow a religious flag to be raised on City Hall Plaza — the same spot they say where banners from “communist” nations as well as transgender and pride colors are set to fly.

“We deserve a voice, too,” said Hal Shurtleff, founder of Camp Constitution, which had its request denied on Sept. 8 by the city after officials argued that it has a practice of “respectfully refraining from flying nonsecular flags” and instead offered Shurtleff an option to fly a nonreligious flag. He declined.

“A lot of people say, ‘Just take no for an answer and walk away.’ I said, no this is our right to do that,” Shurtleff said. “(Flying a Christian flag) does not indicate that Boston is somehow a Christian city. That’s kind of ridiculous.”

Shurtleff and his attorneys said he envisioned holding an event with local clergy in late September to raise the Christian flag, alongside the American flag and city of Boston flag, and discuss the city’s history as well as “our nation’s heritage.”

He said he was motivated to raise the flag — made up of a white background, with a red cross framed by a blue square in the corner — after being invited by the Rev. Bruce Wall to a “prayer meeting” at City Hall in June.

Shurtleff’s group says its mission is to “enhance understanding of our Judeo-Christian moral heritage,” and, according to a letter its attorneys sent the city, has had information tables on the Boston Common “for years,” where it’s distributed copies of the U.S. Constitution.

Shurtleff said he first contacted the city in July. Greg Rooney, the city’s commissioner of property management, sent a written denial Sept. 8 stating that flying the religious flag could send an “overt religious message” and be construed as a city endorsement.

“This policy and practice is consistent with well-established First Amendment jurisprudence prohibiting a local government from ‘respecting an establishment of religion,’ ” Rooney wrote to Shurtleff. “This policy and practice is also consistent with City’s legal authority to choose how a limited government resource, like the City Hall flagpole, is used.”

Shurtleff’s attorneys at the Florida-based Liberty Counsel disagreed, arguing in a letter dated yesterday to Mayor Martin J. Walsh that the city’s decision is unconstitutional. They point to a range of other flags the city deemed appropriate, including ones representing China, Cuba and other countries, as well as the rainbow flag representing LGBT pride, which the Liberty Counsel called the “homosexual rainbow flag.”

The Liberty Counsel has been criticized as being an anti-gay group, which they’ve disputed.

“There is no potential government endorsement, because the flags are private speech,” Richard L. Mast of the Liberty Counsel wrote. “It does not matter what someone subjectively ‘might perceive,’ in terms of true government neutrality (and not hostility) toward private religious expression.”

Mast demanded the city respond by Sept. 27, saying if it doesn’t, Liberty Counsel “will take additional actions to prevent irreparable harm to the rights of our clients.”

The Liberty Counsel this summer filed a motion for summary judgment in a Florida case where a Ten Commandments monument is located next to a county courthouse. An atheist group’s leader is suing the county to take it down.

Constitutional law experts say the city is likely on solid legal ground in denying the request.

“The city’s choice of which flags to fly is likely to be government speech, and thus within its discretion, unless the city really has a policy of accepting all nonreligious flags without any real review,” said Eugene Volokh, a UCLA law professor.

Mark Tushnet, a Harvard law professor, said the city’s interpretation that flying the flag on a city flagpole could be viewed as an endorsement would, in his opinion, hold water with a judge.

“I think it’s much more likely for people to think if it’s on the city flagpole, then the city must be standing behind it,” he said. “If that’s right, then the city in my view is entitled to say no.”