In May 2017, a trial court ruled in their favor and struck down the Department of Revenue’s rule. But in December 2018, the Montana Supreme Court reversed the decision, citing what is commonly known as Montana’s Blaine Amendment. Enacted during a wave of anti-Catholic bigotry in 1889, the amendment bans “direct or indirect” public funding for any “sectarian purpose.” According to the majority opinion, the tax-credit scholarship program violates the Blaine Amendment, even though it’s funded entirely through voluntary, charitable, private donations incentivized by the state’s tax credit. This past summer, the U.S. Supreme Court agreed to hear the case, and it will be argued on January 22.

Read: Your neighbor’s Christian education, courtesy of your tax dollars

Today, 37 states have Blaine Amendments in their state constitution, though the precise wording varies. The amendments are named after Representative James G. Blaine of Maine, who in 1876 proposed a federal constitutional amendment that would have prohibited state funding for schools “under the control of any religious sect.” Blaine’s amendment passed the U.S. House of Representatives but fell just short of the supermajority it needed to pass the U.S. Senate.

Even though Blaine’s amendment was never enacted nationwide, Congress later required many potential states to adopt a version of the amendment in order to be admitted to the union. Starting with the Enabling Act of 1889, Congress granted statehood to Montana, North Dakota, South Dakota, and Washington only after their state constitutions guaranteed that their public schools would be “free from sectarian control.” Senator Henry Blair of New Hampshire, who regularly reintroduced the Blaine Amendment in subsequent sessions, called this provision of the Enabling Act “the very essence” of his proposal, and praised Congress for setting a “great precedent.”

At first glance, Blaine Amendments may seem like a benign way to ensure the government’s neutrality toward religion. In the 1875 speech that inspired Representative Blaine, President Ulysses S. Grant called for a federal ban on government funding for “sectarian schools” in order to “keep the Church and state forever separate.” More recently, nine states with Blaine Amendments collectively filed an amicus brief in the Espinoza case, arguing that no-aid provisions like theirs and Montana’s merely “sought to solidify the Framers’ original design separating church and State.”

But Blaine Amendments weren’t truly devoted to the separation of Church and state. Instead, they were mainly focused on separating the Catholic Church and state. Although public schools are largely secular today, that wasn’t the case in the 19th century. Public or “common” schools typically instilled in their students a nondenominational form of Protestantism, requiring them to sing hymns, pray, and read from the King James Bible—in direct conflict with Catholic dogma.