DEPENDING ON whom you ask, the Supreme Court last week blew a hole in the wall between church and state — or issued a modest decision that calls for little more than reasonableness when the government interacts with religious groups. Who’s right depends on what the court does from here and whether the justices can adopt principles that allow for some curbs on public money flowing into religious activities.

The court considered the case of the Trinity Lutheran Church Child Learning Center, a Missouri preschool that was denied state funds to upgrade its playground surface, replacing coarse pea gravel with recycled tire rubber. Though Trinity Lutheran was near the top of the list of potential nonprofit grant recipients, a provision in the Missouri Constitution appeared to bar public grant money from going to religious institutions.

Affirming that “denying a generally available benefit solely on account of religious identity imposes a penalty on the free exercise of religion,” the court repudiated the state’s grant distribution policy. It “puts Trinity Lutheran to a choice: It may participate in an otherwise available benefit program or remain a religious institution,” Chief Justice John G. Roberts Jr. wrote for the majority. Concurring, Justice Stephen G. Breyer compared the state’s decision to cutting off churches from basic public safety services such as police and fire protection.

The question, then, is whether the court has signaled that a wide variety of public funding programs, including school vouchers, are constitutionally required to include religious institutions. Its broad language condemning the withholding of generally available funds to church organs such as Trinity Lutheran suggests a wide new legal avenue has opened for religious groups to demand a share of taxpayer money.

In fact, the court sent no such clear signal. Though the justices have ruled that, in this case, public money must flow directly to a church, the threat to the separation of church and state will remain relatively contained so long as the justices live up to some limits embedded in their reasoning.

Noting a past decision in which the court dealt more skeptically with claims from the religious side, Mr. Roberts explained that the court ruled differently when the case concerned the trickier issue of public money funding an “essentially religious endeavor,” such as training to join the Christian ministry. “Here, nothing of the sort can be said about a program to use recycled tires to resurface playgrounds,” the chief justice noted. Picking up on this line of thinking, Mr. Breyer wrote, “I find relevant, and would emphasize, the particular nature of the ‘public benefit’ here at issue,” stressing that he “would leave the application of the Free Exercise Clause to other kinds of public benefits for another day.”

Between Mr. Roberts’s and Mr. Breyer’s words, a reasonable principle is identifiable: The government cannot deny churches public funding merely because they are churches, but the government may deny them funding if they would use it for religious endeavors. This principle should guide courts in future cases.