- The case pits free speech, religion and artistic freedom against anti-discrimination laws.
- The gay couple said they were emotionally devastated and felt humiliated after Jack Phillips turned them down in a brief meeting at the bakery.
- Colorado law prohibits discrimination on the basis of race or sexual orientation, and both the state civil rights commission and an appeals court ruled against Phillips, sending the case to Washington.
The US Supreme Court appeared sharply divided on Tuesday as it weighed impassioned arguments in the closely watched case of a Colorado baker who cited his devout Christian faith in refusing to make a wedding cake for a gay couple.
The case pitting free speech, religion and artistic freedom against anti-discrimination laws is the most significant for gay rights since the high court approved same-sex marriage two years ago.
Supporters of both sides gathered outside the court and inside the crowded chamber as lawyers presented their arguments to the nine Supreme Court justices during a 90-minute hearing.
The landmark case pits gay couple Dave Mullins and Charlie Craig against Colorado bakery owner Jack Phillips, who refused in July 2012 to make a cake for their wedding reception.
The four liberal-leaning justices appeared to favour the argument that Phillips, owner of Masterpiece Cakeshop, should be obliged to provide service to all customers, regardless of sexual orientation.
Colorado law prohibits discrimination on the basis of race or sexual orientation, and both the state civil rights commission and an appeals court ruled against Phillips, sending the case to Washington.
The conservative members of the court appeared to be more receptive to the argument that Phillips, who describes himself as a "cake artist," should not be forced to create something that conflicts with his religious beliefs.
Justice Anthony Kennedy, who wrote the 2015 opinion that sanctioned same-sex marriage, is considered the crucial swing vote in the case, which the court is expected to rule on before the end of June.
Kennedy, who generally sides with the conservative faction, had sharp questions for both sides, but he seemed to put great emphasis on Phillips' First Amendment right to free speech.
The first question taken up by the court was whether a cake can be considered artistic expression.
"The primary purpose of a food of any kind is to be eaten," said Justice Sonia Sotomayor, one of the liberal members of the court.
Kristen Waggoner, an attorney for Phillips, countered that his elaborate wedding cakes were more than just food.
"That cake expresses a message," Waggoner said.
"The First Amendment prohibits the government from forcing people to express messages that violate religious convictions."
"Yet the commission requires Mr Phillips to do just that, ordering him to sketch, sculpt and hand-paint cakes that celebrate a view of marriage in violation of his religious convictions," she said.