Washington - The US Supreme Court is to hear arguments on Tuesday in a case that has been described as the most significant for gay rights since it approved same-sex marriage two years ago.The landmark case pits a gay couple, Dave Mullins and Charlie Craig, against a Colorado bakery owner who refused in July 2012 to make a cake for their same-sex wedding reception.Jack Phillips, owner of "Masterpiece Cakeshop" in Lakewood, Colorado, cited his devout Christian beliefs in turning down their request for a cake."What a cake celebrating this event would communicate was a message that contradicts my deepest religious convictions," Phillips wrote this week in USA Today."And as an artist, that's just not something I'm able to do," he said. "So I politely declined."Mullins, a 33-year-old poet and musician, and Craig, a 37-year-old interior designer, said they were emotionally devastated by the rejection and filed suit for discrimination."This case is not about artistic freedom," Mullins told AFP. "We didn't ask for a piece of art."We were simply turned away because of who we were," he said. "We were publicly humiliated for who we were."'Serve the public equally'The Colorado Civil Rights Commission and the state appeals court backed up Mullins and Craig. And now it's in the hands of the nine-member Supreme Court which features a conservative justice, Neil Gorsuch, newly appointed by President Donald Trump.Mullins, Craig and civil rights groups have warned that if Phillips is victorious, other businesses could cite religious beliefs to refuse service to gay customers."When you open a business to the public, you must serve the public equally," Mullins said."If a business owner is allowed to turn someone away just for their strongly held beliefs, could a hotel owner turn away an interracial couple because their faith believes that the races where not meant to mix?" he asked."A loss at the Supreme Court could open the door to many forms of discrimination that have long been considered wrong in our society," he said.'One man and one woman' Phillips said he would be happy to sell Mullins and Craig a cake - just not a wedding cake that would conflict with his conviction that "God designed marriage as the union of one man and one woman"."Just as I shouldn't be able to use the law to force others to design something that promotes my beliefs, others shouldn't be able to force me to design a cake that celebrates theirs," he added in USA Today.Jeremy Tedesco, an attorney for Alliance Defending Freedom, a Christian legal aid group representing Phillips, rejected the argument that the bakery owner's stand is discriminatory."Jack offered to sell the two gentlemen suing him basically anything in his shop," Tedesco told AFP. "Jack serves all who walk through his doors, no matter what their background or walk of life."But like other artists, he just can't create all messages for all events," he said. "Declining to express celebratory messages about same-sex marriages is simply not class-based discrimination."This case is about the freedom of artists and creative professionals to not be forced by the government to create custom expression or work for a message or event that violates who they are," Tedesco said."To say that a Muslim, Jewish, or Christian artist can no longer adhere to the teachings of their faith and be forced by the government to create expression that violates their beliefs is scary," he said."Jack and his family have faced death threats and government punishment," he said. "In a truly free society, tolerance should be a two-way street."'This isn't about a cake'Some 20 states, dozens of members of Congress and Christian lobbyist groups have thrown their weight behind the baker.The Trump administration has also argued that his cakes are a form of artistic expression and that he cannot be forced to use his talents against his own religious beliefs.Among those representing Mullins and Craig is the American Civil Liberties Union, or ACLU."This isn't about a cake," said Louise Melling, the ACLU's deputy legal director."This is a question about whether the Constitution protects the right to discriminate," Melling said."It's about whether the Constitution protects the right of a bakery to put up in its store window a sign that says 'Wedding cakes for heterosexuals only,'" she said.