Today, the U.S. Supreme Court denied a petition asking whether a transit authority can reject a Christmas ad for display on its buses just because the ad is religious. This is an easy question, and it’s a shame the Court denied the petition. Justices Gorsuch and Thomas, though, did write a short consolation prize, saying what they would have said if they granted the case: namely, the government can’t discriminate against a religious viewpoint on a topic while allowing other non-religious viewpoints.

The sides of buses are a frequent and heated battleground for free speech. Transit authorities often draw revenue by selling blank space on their buses. In this case, Archdiocese of Washington v. Washington Metropolitan Area Transit Authority, the Catholic Church tried to place a Christmas ad on D.C. buses with the silhouettes of a few shepherds and the phrase “Find the Perfect Gift.” The transit authority rejected the ad.

The key fact here was that the transit authority allowed other ads about Christmas. All the parties, and the various courts, agreed that Christmas has a “secular” component and a “religious” component. Hence, Wal-Mart and Macy’s and every other retailer could slap their ads on buses across the metropolitan area clamoring about how to celebrate the holiday (by buying their stuff). But a religious advertiser could not express their views on how to celebrate the holiday in that same space, the only difference being the religious nature of the content.

The Supreme Court has repeatedly stated in other settings that similar restrictions constitute viewpoint discrimination. If the government allows speech on a particular subject matter, it cannot then restrict speech on that topic simply because the viewpoint is religious. That’s true even if the proposed speech drips with religious sentiment–such sentiment deserves equal footing under the First Amendment.

This isn’t to say that D.C. buses can now be overrun with religious zealotry. D.C. could lawfully limit advertisements to only commercial ads (they don’t). And of course they could always just forego the revenue and say no ads at all. But if the government opens up a space for expression, it must do so even-handedly.