Oklahoma’s Department of Education ordered every teacher in the state to have a Bible in their classroom and to teach from it, in an announcement on Thursday that challenges U.S. Supreme Court rulings that have found state sponsorship of religion to be unconstitutional.

Ryan Walters, Oklahoma’s superintendent of public instruction, announced the order with immediate effect at Thursday’s Department of Education board meeting, in which he said special attention will be afforded to the Ten Commandments.

“Every teacher, every classroom in the state will have a Bible in the classroom, and will be teaching from the Bible in the classroom to ensure that this historical understanding is there for every student in the state of Oklahoma,” he said.

He called the Bible, the holy scriptures of Judaism and Christianity, one of the “foundational documents of … Western civilization.” He said important historical figures, including civil rights leader Martin Luther King, Jr., referred to the text.

Both the Hebrew and Christian Bible include the Jewish prophet Moses receiving the Ten Commandments on Mount Sinai, while only the Christian Bible includes the New Testament. Walters, who is Christian, did not stipulate which version teachers must use to comply with his order, and his spokesperson declined to answer questions.

The Establishment Clause of the U.S. Constitution’s First Amendment has been interpreted to prohibit the state from sponsoring or establishing any particular religion. The Oklahoma Constitution goes further, stipulating that any public school and spending of public funds must be nonsectarian, and not benefit “any sect, church, denomination, or system of religion.”

That part of the state constitution was cited two days before Walters’ announcement, when the Oklahoma Supreme Court struck down an effort in which Walters was involved to create the first taxpayer-funded religious charter school in the U.S.

The main teachers’ labor union in Oklahoma said Walters’ Bible order was unconstitutional and that state law said school districts have the right to decide which books are available in their classrooms.

“Teaching about the historical context of religion (and the Bible) is permissible; however, teaching religious doctrine is not permissible,” the Oklahoma Education Association said in a statement. “Public schools cannot indoctrinate students with a particular religious belief or religious curriculum.”