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When David Mullins and Charlie Craig walked into Masterpiece Cakeshop, a bakery in Denver, Colorado, five years ago, they had no inkling that the encounter would take them to the United States Supreme Court. All they wanted was a wedding cake. But as soon as Jack Phillips, the bakery’s owner, realized that the marriage they were celebrating was their own, he cut off the conversation, explaining that he would not make any cake for a same-sex wedding. They never even discussed what the cake would look like or say, because Phillips made it clear that his policy was absolute. The bakery has turned away several other same-sex couples on the same grounds, including a lesbian couple who wanted to buy cupcakes for a commitment ceremony. Phillips claims that because he objects to same-sex unions on religious grounds, and because his cakes are a form of expression, he has a First Amendment right to refuse to sell them to gay couples for their wedding receptions.

When they were turned away, Mullins and Craig brought a complaint before the Colorado Civil Rights Commission, which enforces the state’s public accommodations law. That law, which dates back to 1885, requires businesses open to the public to treat their customers equally. (Forty-five states have a similar law, as does the federal government.) Since 2008, Colorado has specifically prohibited businesses from discriminating against customers on the basis of sexual orientation, in addition to disability, race, creed, color, sex, marital status, national origin, and ancestry. The commission found that by selling wedding cakes to straight couples but refusing to sell them to gay couples, the bakery had violated the public accommodations law. The Colorado courts upheld that decision, rejecting the bakery’s First Amendment objections—as have courts hearing similar cases involving florists, banquet halls, photographers, and videographers. In June, however, despite the unanimity among the lower courts, the Supreme Court agreed to hear the bakery’s appeal.

The case will be argued on December 5. (The ACLU represents Craig and Mullins, and I am co-counsel in the case.) It asks whose rights should prevail when the First Amendment’s guarantees of freedom of religion, speech, and association come into conflict with equality—over a cake, no less. It is one of the most talked-about cases of the term, in part because it’s so easy to conjure hypothetical variations: What if the cake includes the message “God bless this union”? What if a wedding photographer, who has to be present at the ceremony in order to provide her services, objects to same-sex marriage? Should bakeries or photographers be permitted to refuse their services to an interracial or interfaith couple? Could a bakery refuse to make a birthday cake for a black family because its owner objects to celebrating black lives?

The Masterpiece Cakeshop dispute is at the center of a new front in the clash between opponents and proponents…