The central issue is whether the religious expression is properly attributed to the government or to private speakers. Where the government itself is “speaking,” the establishment clause generally prohibits religious messages, and the free speech clause generally does not apply. But where private speech is involved, establishment clause concerns are diminished, and the free speech clause requires the government to treat all speakers equally.

We argue that no reasonable observer would understand flying Camp Constitution’s flag — for just one hour on a single day — to be the government’s speech. Like the 284 flags flown before it, this group’s flag would be seen as just that — the group’s flag. And as such, the city can’t turn it down because the flag is religious.

The U.S. Court of Appeals for the 1st Circuit nonetheless ruled for the city last January, concluding that the flag displays did amount to government speech — so the city could choose which flags to display as its own speech, without regard to the rights of the private flagholders. On this theory, the city could also have refused to display the Boston Pride flag, or any other.

But that is a dangerous expansion of the “government speech” doctrine.