He runs his business guided by religious principles, closing on Sunday and refusing to make cakes containing alcohol or celebrating Halloween.
But the Supreme Court, bolstered last April by the addition of stalwart conservative and fellow Coloradan Neil Gorsuch, represented a tougher test.
Kennedy said in his majority opinion that the larger issue "must await further elaboration" in the courts.
Baker Jack Phillips had argued that cake-baking is constitutionally protected free speech and that sanctioning him for refusing to bake for a same-sex marriage violated his constitutional right to free exercise of religion. "T$3 he only reason the Commission seemed to supply for its discrimination was that it found Mr. Phillips's religious beliefs 'offensive, '" the concurrence noted.
The Colorado case started when same-sex couple Charlie Craig and Dave Mullins visited Phillips' Masterpiece Cakeshop in suburban Denver for a wedding cake.
In 2012, Dave Mullins and Charlie Craig were planning a celebration after they were married in MA. After the couple was denied, they filed discrimination charges with the Colorado Civil Rights Commission, which agreed with Craig and Mullins.
"The commission's hostility was inconsistent with the First Amendment's guarantee that our laws be applied in a manner that is neutral toward religion", Kennedy wrote. Justice Anthony Kennedy authored the majority opinions in both cases.
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"It must be emphasized that religions, and those who adhere to religious doctrines, may continue to advocate with utmost, honest conviction that, by divine precepts, same-sex marriage should not be condoned", Kennedy wrote in 2015.
The ruling (pdf) was 7-2, with Justices Ruth Bader Ginsburg and Sonia Sotomayor dissenting.
The Supreme Court ruled unanimously against the Hialeah ban.
The Supreme Court is expected to decide this month whether or not it will accept the case, or send it back to the Washington State Supreme Court for another review.
When arguments were held before the court in December, President Donald Trump's administration spoke in support of Phillips. "The First Amendment prohibits governments from discriminating against citizens on the basis of religious beliefs".
"We are pleased with today's Supreme Court decision", Sessions said in a statement.
Waggoner and Esseks disagreed about the ruling's effect on Phillips' wedding cake business.
Phillips believes that same-sex marriages are sinful, that marriage is to be between a man and a woman. For example, he refused to bake cakes that celebrated divorces, cakes that were infused with alcohol, cakes with obscene language or artwork, or cakes celebrating same sex weddings. "What a cake celebrating this event would communicate was a message that contradicts my deepest religious convictions, and as an artist, that's just not something I'm able to do, so I politely declined", he said. State officials joined the case against Phillips, and a Colorado appeals court ruled against the bakery.