As to Mr. Phillips’s free exercise of religion claim, the Supreme Court has said that the First Amendment is not a license to discriminate in the face of neutral, generally applicable laws like Colorado’s. In 1968, a few years after the Civil Rights Act passed, the court ruled unanimously against the owner of a South Carolina barbecue chain who invoked his religious freedom to refuse to serve black people. The act “contravenes the will of God,” he claimed. The court called that argument “patently frivolous.”

Recognizing, perhaps, the weakness of the religious-freedom argument, Mr. Phillips now emphasizes his other First Amendment rights — freedom of speech and expression. His cakes are his artistic expression, he says, and he should not be forced to express ideas to which he is opposed.

Mr. Phillips makes a good case that he is an artist. So might many others who sell the fruits of their labor to those celebrating a wedding. But that doesn’t give any of them the right to refuse service to people protected under an anti-discrimination law. If the couple had asked Mr. Phillips to write a message on their cake endorsing same-sex marriage and he had been punished for refusing, he would have a more plausible First Amendment claim, since he wouldn’t write that for anyone. But Colorado’s law doesn’t compel Mr. Phillips, or any proprietor, to say anything they don’t want to say, or to endorse any specific message. It requires only that they treat all customers equally.

Mr. Phillips claims he already does this. He’s happy to sell any of his pre-made products to gay people, he says, or to bake them a custom cake for another occasion. What he won’t do is custom-bake anything intended for use in a same-sex wedding. As the Colorado Civil Rights Commission said in ruling for Mr. Mullins and Mr. Craig, that’s a distinction without a difference. Since only gay people have same-sex weddings, he’s discriminating against gay people.

Some free-speech advocates argue that this case is simply a matter of deciding which sorts of expression merit First Amendment protection and which do not. Cake bakers may be a close call, but what about photographers? Florists? Caterers? Calligraphers? In fact, cases like these have already been brought around the country. If the justices rule for Mr. Phillips, they will be hard-pressed to find a clear limiting principle. And that would render public-accommodations laws like Colorado’s effectively meaningless.

This, of course, is precisely the objective of the rear-guard action undertaken by religious objectors who, thwarted in their efforts to prevent gay couples from enjoying the rights and benefits that flow from marriage, are now invoking their own constitutional rights to avoid treating those same couples equally in the marketplace.