A unanimous Supreme Court ruled Monday that Boston violated the free speech rights of a conservative activist when it refused his request to fly a Christian flag on a flagpole outside City Hall.

Justice Stephen Breyer wrote for the court that the city discriminated against the activist, Harold Shurtleff, because of his “religious viewpoint,” even though it had routinely approved applications for the use of one of the three flagpoles outside City Hall that fly the U.S., Massachusetts and Boston flags.

Occasionally, the city takes down its own pennant and temporarily hoists another flag.

Shurtleff and his Camp Constitution wanted to fly a white banner with a red cross on a blue background in the upper left corner, called the Christian flag, to mark Constitution Day, Sept. 17, in 2017.

The city had approved 284 consecutive applications to fly flags, usually those of other nations, before it rejected Shurtleff’s because it was a Christian flag. The city said he could fly a different banner, but Shurtleff refused, and lower courts upheld the city’s decision.

But the high court said the lower courts and the city were wrong. The case hinged on whether the flag-flying is an act of the government, in which case Boston can do whatever it wants, or private parties like Shurtleff, Breyer wrote.