This week marks a key hearing in a U.S. Supreme Court case that could have drastic implications for our LGBTQ residents. The case, which concerns a Colorado bakery that refused to serve a same-sex couple, threatens the very freedoms on which this country was founded. As co-chairs of the national Mayors Against LGBT Discrimination coalition, we are proud to join more than 150 other mayors and municipalities nationwide in opposing religious exemptions that allow sexual orientation-based discrimination.

The case at issue, Masterpiece Cakeshop v. Colorado Civil Rights Commission, began when a Colorado business refused a same-sex couple that wished to order a wedding cake. In doing so, the business claimed that religious beliefs – beliefs opposing same-sex marriage – justified this refusal. In 2013, a federal court ruled against the business, stating that the business' actions violated Colorado's nondiscrimination law. The Supreme Court heard oral arguments in the case on Tuesday and will issue its ruling this spring.

As the mayors of three of America's biggest and most diverse cities, we serve people who represent all backgrounds and belief systems. We strive to create communities where everyone is treated with dignity, fairness, and respect. When we allow anyone a license to discriminate, we set the stage for further discrimination surrounding sexual orientation, gender identity, race, religion, sex, and more. Most of all, we undermine the promise that we made to our residents – a promise that in our cities, they will enjoy equality in every respect.

We cannot let discrimination flourish in our cities or in our country. Throughout history, this nation has overturned exceptions that let businesses turn away certain people. Through efforts that included the civil rights, gender rights, and disability movements, we have embraced a powerful ideal: When a business is open to the public, it must be unconditionally and equally open to all. Despite progress on many fronts, LGBTQ people continue to face daily attacks and hostility. Now, more than ever, we cannot open the floodgates to further discrimination.

Mayors Against LGBT Discrimination was launched to support our LGBTQ residents. This coalition, formed in January, exists to support nondiscrimination protections – and to oppose sweeping exemptions that are based on religious beliefs. More than 250 mayors have joined these efforts and are proactively supporting their LGBTQ residents through community forums, media statements, and partnerships with local liaisons. More than 200 municipalities and 18 states already have nondiscrimination laws, much like the one in Colorado, allowing people to access businesses and services without fear. We know that these laws work, and we must play a role in upholding and bolstering these protections.

This nation was founded on a promise that all Americans should enjoy the values of freedom, fairness and equal treatment. We believe that our Constitution enshrined these promises into law, ensuring that future generations would continue to enjoy these protections. We hope the Supreme Court will remember these ideals and that it will uphold the very freedoms that define this nation.

Mayor Kenney of Philadelphia (@PhillyMayor), Mayor Ed Lee of San Francisco (@mayoredlee), and Mayor Muriel Bowser of Washington, D.C. (@MayorBowser), are the co-chairs of Mayors Against LGBT Discrimination.