The substance of a Supreme Court decision is occasionally less revealing than what it says implicitly about the state of the national dialogue. And a good example is the decision in the case of the after-hours ''Christian club'' at an Omaha high school. It offers dismal witness to what years of violent controversy have done to the Establishment Clause, and to religion and learning.

Six years ago, Congress passed a so-called ''equal access'' law. Its essence is that when schools allow the usual array of after-hours clubs and groups, they may not ban, for ideological or philosophical reasons, a club whose primary purpose is religious study.

A year later, some students in Omaha asked to be allowed to start a club devoted to Bible reading, discussion and prayer. It would meet at school, but after hours -- like the chess and scuba-diving clubs. School authorities balked. They said it was not ''curriculum-related,'' and that even if it were, it would violate the principle of church-state separation.

This view was seriously in error, and the court has corrected it.

But the argument about the Omaha ''Christian club'' is symptomatic of larger, and contagious, absurdities. The notion that a student Bible-study club that is not officially sponsored, and is entirely voluntary might somehow be an illicit ''establishment of religion'' is farfetched. It springs from a gross but common misreading of what the court has said over the years about prayer and Bible reading in the schools.

It was not study, it was official prescription of prayer and Bible reading in devotional settings, that the court sought to check; and properly so. Otherwise, and as properly, the court has never attempted to dictate rules for the study of religion, even in the classroom, let alone after hours. Indeed, in the school prayer decision, the court went out of its way to note that it was not attempting to expunge religious or scriptural study.

But that disclaimer, along with other subtleties, soon was lost sight of. And no wonder. The shrill criticism that followed the prayer decision had an explosive theme: The court was ''taking God out of the schools'' or ''secularizing'' them. These misconceived and mischievous charges took their toll. They confused school boards and administrators, textbook publishers, parents and students, and even an occasional lower-court judge. Religion became too hot to handle, too controversial to be dealt with in a sane and balanced way. The easy alternative was to pretend it was nonexistent or unimportant. Hence the absurdities multiplied.

People for the American Way, for instance, discovered not long ago that in their usual mousy fashion, certain textbook publishers had laundered the religious factor right out of American history. It was ''Hamlet'' without the ghost. It was as if Massachusetts Bay or Rhode Island had begun as overseas health resorts for the 17th century equivalent of the English jet set. And what odd contortions were resorted to in the teaching of literature. What might be said, for instance, about Milton's ''Paradise Lost'' -- on the premise that the inspirations of religion were an unmentionable subject -- could only be imagined.

The court's internal argument in the Omaha case reflects the same reticence, the same fear of grappling incisively with the central issue. Eight of the nine justices agreed that the ''equal access'' law does not conflict with the First Amendment, but the varied explanations commanded no single majority. Meanwhile, the great question of how the vital protections of the Establishment Clause may be reconciled with a sensible acknowledgment of the centrality of religious tradition in our history or in Western culture went unaddressed. One could only imagine, rather wistfully, what use a Felix Frankfurter or a Robert Jackson would have made of such an invitation to reflection.

No, the court's rather dreary argument mainly had to do with the meaning of the term ''noncurricular,'' with the dangers that ''peer pressure'' might drive students against their will into the Christian club, and the like. But why is it so hard to distinguish between worship or indoctrination in the schools (which are out of place) and the study of religion (which is not)? Why can't we agree to keep sectarian rituals out of the classroom without making religion into a bugaboo?

The court is a great teaching, as well as adjudicating, institution. But on this subject, it has turned the classroom over to the dunces and zealots. And with the results you might expect.