The eleventh commandment for the Supreme Court appears to be: "Thou shalt not be clear." To put it charitably, the justices left Ten Commandments jurisprudence in a state of confusion last June. A majority held unconstitutional a courthouse display in Kentucky, but sustained a display at the Texas Capitol. Seemingly every justice rendered a separate opinion.

Legal scholars, it is true, have grown accustomed to opaque decisions handed down by a closely divided Court. The trouble is, the Court does not—or should not—write for the scribes. It writes to offer guidance to a nation. In that role, it has the obligation to craft clear rules that legislatures, governors, school boards, and federal agencies can apply in the real world.

The First Amendment prohibits any establishment of religion. The Supreme Court has struggled for more than half a century to figure out just what that means. Sometimes the results have been sensible, if controversial. For example, I think the justices were correct in holding that the amendment bans organized prayer in public school classrooms.

Sometimes, however, the results have seemed, even at second and third glance, almost silly. For example, after years of teaching and writing on the subject, I still cannot fathom the Court's conclusion a decade and a half ago that prayer at a high-school graduation is more threatening than, say, prayer at the opening of a legislative session.

A quarter-century ago, the justices held unconstitutional a display of the Commandments on the wall of a schoolroom. Ever since, lower courts have struggled over where else the dangerous words ought to be banned. At the same time, communities determined not to drown in the rising tide of secularism have struggled over ...