On Tuesday, the Supreme Court hears oral arguments in Masterpiece Cakeshop v. Colorado Civil Rights Commission ­— the case of the same-sex wedding cake and the baker who refused to make it. A state agency ruled that the baker, Jack Phillips, was in violation of Colorado’s anti-discrimination laws and decreed that if he wishes to create wedding cakes at all, he must create them for same-sex weddings too.

But Masterpiece Cakeshop is not about gay marriage. It’s about compelled speech.

The Supreme Court settled the marriage issue in its landmark Obergefell decision in 2015. Gay and lesbian couples are free to marry anywhere in the United States, and government at every level now protects their right to do so. But can government require artists, designers, or other creative professionals to celebrate same-sex marriage through their work? Can it subject someone like Phillips — who will happily serve any customer but cannot in good conscience use his cake-design skills to communicate an endorsement of gay marriage — to prosecution, sanctions, or legal coercion?

Most Americans support same-sex marriage. A sizable minority does not. In Obergefell, the high court emphasized that dissent is entirely legitimate. “Many who deem same-sex marriage to be wrong reach that conclusion based on decent and honorable religious or philosophical premises, and neither they nor their beliefs are disparaged here,” Justice Anthony Kennedy wrote in his majority opinion. While same-sex couples would no longer be barred from marrying, he added, nothing should impede the continuation of “an open and searching debate.”

Were the justices serious about that? If so — if they truly don’t want the legalization of same-sex weddings to become an excuse to persecute “decent and honorable” Americans who oppose gay marriage — they will use this case to say so. They’ll reverse the decision of the Colorado courts and uphold Phillips’s right not to support a practice he believes is wrong.

For Kennedy in particular, this case offers an exquisite opportunity to uphold two cherished principles: first, that the benefits of marriage not be denied on the basis of sexual orientation, and second, that liberty is threatened most when government seeks to control thought or speech.

In America, the state cannot force citizens to express a certain point of view. In 1943, in one of its most famous decisions, the Supreme Court ruled that West Virginia schoolchildren could not be compelled to salute the flag: “If there is any fixed star in our constitutional constellation,” Justice Robert Jackson declared, “it is that no official, high or petty, can prescribe what shall be orthodox in politics, nationalism, religion, or other matters of opinion or force citizens to confess by word or act their faith therein.”

But is refusing to create a custom-designed wedding cake, a skeptic might ask, really comparable to not saluting the flag? After all, the latter is an explicit demonstration of political loyalty; the cake is just — dessert.

Yet by that logic, a painting is just décor. A song is just entertainment. Calligraphy is just fancy lettering.

That’s a dangerous argument — dangerous to the liberty of mind and conscience that the First Amendment shields. One of the many friend-of-the-court briefs filed in this case was submitted by 479 creative professionals representing all 50 states; the group comprises musicians, florists, videographers, ceramic artists, calligraphers, graphic designers, cartoonists, sculptors, and painters. Their brief urges the high court to defend the First Amendment rights of “artistic expression — regardless of the medium employed.” They make a vital point: Viewpoints and messages can be expressed in many forms, and the Bill of Rights protects them all.

This isn’t a new idea in First Amendment jurisprudence. The Supreme Court itself underscored it in a landmark case out of Boston — the 1995 St. Patrick’s Day parade controversy.

In Hurley v. Irish-American Gay, Lesbian & Bisexual Group of Boston, the justices ruled unanimously that the parade’s organizers, a South Boston veterans council, could not be compelled to include an LGBT group seeking to express pride in both its Irish heritage and its gay identity. The First Amendment trumped the state’s anti-discrimination statute, the court held, and the government could not force the veterans to express, through their parade, a message they didn’t agree with.

Hurley is the precedent most directly applicable to the case before the court this week, both for its resounding rejection of compelled speech and for its insistence that expression is not limited to writing and speaking.

“The Constitution looks beyond written or spoken words as mediums of expression,” Hurley affirmed. “Our cases have recognized that the First Amendment shields such acts as saluting a flag (and refusing to do so), wearing an armband to protest a war, displaying a red flag, and even … displaying the swastika.” The Constitution doesn’t protect only banners and speeches; otherwise, the First Amendment “would never reach the unquestionably shielded painting of Jackson Pollock, music of Arnold Schoenberg, or Jabberwocky verse of Lewis Carroll.”

Or the custom-designed edible artwork of baker Jack Phillips.

One needn’t share Phillips’s opinion of gay marriage to support his right to unmolested freedom of expression. Indeed, some groups that vigorously oppose his beliefs about matrimony have filed amicus briefs on his behalf. The right to have views that others don’t share, they know, is a quintessential American liberty.

And the right not to be prosecuted for expressing those opinions is another.

Jeff Jacoby is a columnist for The Boston Globe.