Since nobody can agree on what that means, Americans have spent the last three decades debating and litigating who gets to pray - and what they can say - without running afoul of the court's murky guidance.
On May 20, the Supreme Court agreed to hear a case, Town of Greece, NY v. Galloway, which may finally clear up some of the confusion surrounding the constitutionality of saying prayers at the opening of a government meeting.
Short of reversing Marsh by prohibiting opening prayers at legislative sessions which this Court is very unlikely to do, the only fair alternative would appear to be a "prayer policy" that includes everyone by rotating among the bewildering variety of faiths and beliefs represented in the most religiously diverse society in the world.
Or the Court could mandate general, universal "to-whom-it-may-concern" prayers that, in the end, satisfy no one (and exclude the nonreligious).
What the Supreme Court must not do, however, is allow any government body to endorse one faith over others by opening meetings with Christian prayers week after week. If "no establishment" under the First Amendment means anything, it means at least this: The government may not take sides in religion.
Charles C. Haynes is director of the Religious Freedom Education Project at the Newseum.