In a win for the separation of church and state, the Oklahoma Supreme Court ruled that Oklahoma’s approval of the nation’s first religious public charter school violates the state constitution and charter-school statute, as well as the U.S. Constitution. The decision affirms what we already knew: A religious school can’t be a public school, and a public school can’t be religious.

Last year, St. Isidore of Seville Catholic Virtual School applied to the Oklahoma Virtual Charter School Board to become a public charter school. The school, which would have been managed by the Archdiocese of Oklahoma City, proclaimed in its application that it would carry out “the evangelizing mission of the [Catholic] Church” by fully embracing its religious teachings and incorporating those teachings “into every aspect of the School.” The school also acknowledged that it would discriminate in admissions, student discipline, and employment, as necessary to satisfy the Catholic Church’s religious doctrine, and that it would not accommodate a student’s disability if doing so would violate the school’s Catholic beliefs.

Despite warnings from the Oklahoma attorney general, education groups, and civil rights organizations that public schools—including charter schools—cannot legally teach a religious curriculum or discriminate against students and employees, the Virtual Charter School Board approved St. Isidore’s application and entered into an agreement allowing the school to begin operating for the upcoming school year. Today, in ordering the state board to rescind its contract with St. Isidore, the Oklahoma Supreme Court sent a pointed message: Our public schools are for education, not evangelizing.

The court held that charter schools, which are funded by the state, created as government entities, and expressly characterized in state law as “public schools,” are, of course, just that – public schools. As a result, the court explained, a religious public charter school violates not only the Establishment Clause of the First Amendment, but also Oklahoma’s charter school law and constitution, which forbid public schools from imposing religious teachings on students. “Enforcing the St. Isidore contract would create a slippery slope and what the [state constitution’s] framers warned against—the destruction of Oklahomans’ freedom to practice religion without fear of governmental intervention,” the court stated.

The ruling comes in response to a petition filed with the Oklahoma Supreme Court by the Oklahoma attorney general, who sought to rescind the Charter School Board’s contract with St. Isidore. Although some people may be surprised that a Republican attorney general would object to the nation’s first religious public charter school, safeguarding the separation of church and state is not, and never should be, a partisan issue.

That’s why the ACLU, along with Americans United for Separation of Church and State, Education Law Center, and the Freedom From Religion Foundation, filed a friend-of-the-court brief in the case supporting the attorney general. Even before the attorney general filed his petition, we brought suit in Oklahoma state court on behalf of parents, faith leaders, and public-school advocates who don’t want their tax dollars used to fund a religious public school that discriminates against students and staff and promotes religious doctrine.

Church-state separation is a cornerstone of our democracy. It’s critical to preserving the right of every person to decide for themselves—without pressure from the government—which religious beliefs, if any, to hold and practice. It also ensures that the government doesn’t undermine religion either by co-opting it for political purposes or rendering religious institutions dependent on the state to spread their faith. Indeed, the U.S. Supreme Court has repeatedly emphasized that the separation between religion and government is particularly crucial in our public schools, which, by design, freely serve all students equally regardless of religious background or preference.

St. Isidore is, and has always been, free to open as a private religious school that taxpayers would not be forced to support. It is not free, however, to assume the mantle of a public school—including all the associated legal and financial benefits—while flouting the Oklahoma and U.S. Constitutions. The Oklahoma Supreme Court recognized as much, explaining, “What St. Isidore requests from this court is beyond the fair treatment of a private religious institution receiving a generally available benefit…It is about the state’s creation and funding of a new religious institution violating the Establishment Clause.”