Something deeply un-American is underway in the state of Oklahoma.

In June, Oklahoma’s Statewide Virtual Charter School Board approved the nation’s first religious public charter school. The Archdiocese of Oklahoma City and the Diocese of Tulsa were given permission to open St. Isidore of Seville Catholic Virtual School in August 2024.

That’s right, a religious public school, funded by the state’s taxpayers. Proponents hope this model will spread to the dozens of other states that allow charter schools. Seven percent of public school students in the country attended charter schools as of the fall of 2021, and that number continues to grow. That’s why Christian nationalist groups see charter schools as fertile ground for their full-on assault on the separation of church and state in public education.

In just the past year, significant progress has been made in infusing Christianity into public schools. Texas, for example, now allows public schools to replace certified school counselors with religious chaplains and came close to requiring every classroom to display the Ten Commandments. New laws in Idaho and Kentucky could allow teachers and other public school employees to pray in front of — and even with — students. Missouri and Louisiana authorized public schools to teach Bible classes. West Virginia nearly passed a bill that would allow public schools to teach intelligent design creationism. Accompanying these laws are increasingly successful efforts to ban books and lessons about race, sexual orientation, gender identity and even menstruation in public schools.

The establishment of a school that claims to be simultaneously public and religious — what has been a legal oxymoron in the United States since its founding — violates one of the foundational principles of American constitutional tradition: the separation of church and state. It also threatens religious freedom and undermines public education.

The United States Supreme Court has emboldened Christian nationalists by holding twice in the past three years that if a state funds private secular schools, it must also fund private religious schools. But charter schools are taxpayer-financed public schools — not private schools.

Oklahoma law stipulates that charter schools are public schools and “shall be nonsectarian in” their “programs, admission policies, employment practices and all other operations.” But late last year, the state’s attorney general at the time, John O’Connor, issued an advisory opinion requested by the executive director of the charter school board, in which he concluded that those restrictions most likely violated the First Amendment’s free exercise clause.

This is the same Mr. O’Connor who, in a speech to a Rotary Club earlier in 2022, expressed concern about a “godless America” and went on to say, according to The Sand Springs Leader, that a God-based country isn’t one in which “everybody is forced to believe the same thing. It means we acknowledge that there’s a God who has values and endows us or imbues us with those values that are not granted to us by the government. They are granted to us by God.”

Mr. O’Connor was defeated last summer in the state’s Republican primary, and his successor, a fellow Republican and the current attorney general, Gentner Drummond, rescinded his predecessor’s advisory opinion, called the charter school board’s decision “unconstitutional” and warned of possible legal action if a contract for the school is signed.

In a recent interview with Politico, Mr. Drummond said that he believed the “genesis” of efforts to have taxpayers pay for religious-based school instruction “is in Christian nationalism.” He told Politico’s Weekly Education newsletter that “this Christian nationalism is the movement that is giving oxygen to this attempt to eviscerate the establishment clause,” the doctrine of separation of church and state.

Indeed, it’s hard to think of a clearer violation of the religious freedom of Oklahoma taxpayers and public-school families than the state establishing a supposedly public school that is run as a religious school. Forcing taxpayers to fund religion, let alone a religion not their own, violates the Oklahoma Constitution’s explicit command that no public money or property “shall ever” be used to benefit or support religion. It’s exactly what Thomas Jefferson labeled “sinful and tyrannical.”

That is why the organization I head, Americans United for Separation of Church and State, together with the A.C.L.U., the Education Law Center and the Freedom From Religion Foundation, filed a lawsuit on Monday in state district court in Oklahoma to prevent the school from operating as a charter school.

Public schools must remain neutral when it comes to religion and must welcome all. St. Isidore claims it welcomes “students of all faiths or no faith.”

But here’s the catch: According to St. Isidore’s website, students must “appreciate and desire a robust Catholic education,” and students and families must have a “willingness to adhere with respect to the beliefs, expectations, policies and procedures of the school.” St. Isidore also said that it will operate “in harmony with faith and morals, including sexual morality, as taught and understood by the Magisterium of the Catholic Church based upon Holy Scripture and sacred tradition.” That’s a clear description of a Catholic school, not a public one.

A public school that is subsumed in any one church’s dogma is no longer a public school. Yet Oklahoma taxpayers will be on the hook to pay for it.

Rachel Laser is the president and chief executive of Americans United for Separation of Church and State.