During the past 18 months, the Federal Communications Commission has been unindated with more than 5 million letters sent by Americans who erroneously believe the commission is considering a petition to ban religious broadcasting.

An average of 8,000 letters each day have been arriving in the commission's mailroom at 1919 M St. NW. The FCC has hired two extra employees and temporarily rented additional space to handle the letters. Last fall, the commission destroyed about 4 million letters after it got permission to do so from the General Services Administration.

Practically all the mail is a form letter. The letter claims that atheist Madalyn Murry O'Hair, who got the Supreme Court to ban prayers and bible readings in public schools, is behind an effort to get religious broadcasts banned from the airwaves.

"We don't know what's behind it," said Roscoe E. Long, chief of the police and rules division of the FCC's broadcast bureau.

"It seems to be some kind of organized effort by well-meaning people who are afraid something is happening, and there is a certain appeal in writing their government about a very sensitive issue in this country: government and religion," he said.

Long said the commission is hesitant about putting out appeals to people to stop writing over radio and TV stations might consider such a request for air time "coercive."

Lately, Long said, he has even had indications that nonreligious groups such as the American Association of University Women and the American Farm Bureau are becoming involved in the mass mailings to the FCC.

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FCC officials say they have tried everything they can think of to dispel the rumor that a petition aimed at ending religious broadcasting exists or that O'Hair, president of American Atheists, is involved in such an attempt.

They have given media interviews, made personal appearances, sent letters to editors, telephoned letter writers, developed lengthy press releases and form letters and sought the cooperation of groups like the National Religious Broadcasters and major religious denominations to inform the public of the truth.

Last July, TV Guide - which reaches 46 million adults each week - carried a cover story called, "The Rumor the FCC Can't Kill," and still the mail comes in. In fact, it is increasing, and no one can explain why, said Long.

O'Hair who lives in Austin, Tex., also is being deluged. "You don't know how many Bibles . . . tape cassettes . . . records and record sets I get from religious nuts who think I'm involved with this," she said.

"I get rosaries in the mail, telephone calls. Just last week we had to physically extrude a protestor from our office. I'm running for City Council and they're trying to inject this into the campaign. I'm starting to make more TV appearances just to say my hands are clean on this issue."

The FCC did consider a petition two years ago that sought some curbs on fundamentalist religious broadcasts. It was vigorously opposed and generated 700,000 comments, more mail than any other subject in FCC history.

The petition filed in December, 1974, by Jeremy D. Lansman and Lorenzo W. Milam, broadcast consultants in California, asked the FCC to "freeze" applications by religious institutions for TV or FM channels that are reserved for educational stations.

The case did not involve religious programming on any commercial network or station, where the preponderance of religious shows exist. Immediately, however, it was misinterpreted, and the deluge of mail began.

The FCC unanimously denied the petition on Aug. 1, 1975, declaring that the First Amendment requires government agencies to be neutral toward religion.

About the same time, the rumor got started that O'Hair was involved, and since then, some of the protesting petitions to the FCC have contained as many as 10,000 signatures.

The form letters charge that "if her (O'Hair's) attempt is successful, all Sunday worship services, currently being broadcast either by radio or television would cease. Many elderly people and shut-ins, as well as those recuperating from illness or hospital visits, depend on radio and television to fulfill their worship needs every week."

It also warns against O'Hair's effort to stop astronauts from praying and reading the Bible in outer space, and attempt that died in the courts in 1971.

Long's staff doesn't answer the mail, except for the hundreds of congressional inquiries about the nonexistent petition, and he said he has no estimate of the cost to taxpayers in staff time devoted to the imaginary issue.

He has computed that postage on the mail received has cost $650,000. "Just think of all the work which could have been done in the churches with the money," he mused.