Justice Holmes, who said our free speech guarantees do not protect anyone who shouts "Fire!" in a crowded theater, said nothing about the evils of race-baiting in a crowded church.

Still, Mayor Marion S. Barry should have known better. But there he was, speaking to some 2,000 local citizens who had come to mourn the 21 dead or missing Atlanta youngsters, suggesting that that nation-shocking tragedy is part of some anti-black conspiracy that involves, at least through their unconcern, the Reagan administration and, by implication, the FBI.

"A certain mood exists in this country, encouraged by the leadership, that it is all right to do anything to black people," Barry told the mourners at Shiloh Baptist Church.

"Now I maintain that if those were 21 white people, we would have no problem [getting the federal government involved]."

The church, according to a reporter on the scene, "exploded with applause."

Maybe Barry really does believe, as some alarmed and frustrated blacks have claimed, that there is a racist conspiracy linking the murders in Atlanta to those in Buffalo and Salt Lake City. Maybe he believes that the killings result from the ascendancy of political conservatives. Maybe he is simply too much of a politician to pass up an opportunity for near-automatic applause.

An maybe he is, like millions across America, merely frustrated and grief-stricken that the authorities do not seem close to solving the Atlanta murders.

It must be terribly easy to take the rehtorical slide from expressing grief that law enforcement efforts have so far not succeeded to the conclusion that the failure must be due to a lack of trying.

But surely Barry, and the others who have voiced similar sentiments, must believe that Atlanta's mayor, Marynard Jackson, and his chief of police, both black, want the murders solved. Surely he doesn't believe that the Atlanta police, who took the extraordinary step of asking assistance from crack detectives, black and white, from across the nation, don't want to put an end to the gruesome murders.

As far as the national government is concerned, surely it must mean something that the FBI has made its laboratory facilities, its identification facilities and as many as 100 agents -- 30 of them on the scene in Atlanta -- available to help in cracking the case.

What purpose can it serve to suggest that nobody is seriously trying to find the killer(s) -- or even to conclude so categorically that since the victims are black, the killer must be white?

Why can't we just assume that the Atlanta slayings constitute a hellishly difficult case -- as did the San Francisco "Zebra" murders (whose victims were white) or the murders of 33 white boys in Chicago, whose deaths had not even been linked until the authorities lucked upon John Wayne Gacy?

It must be frustration, just as it must have been frustration that led some of the mourners at Shiloh to groan when another speaker, a black bishop, suggested, "Now, perchance, if one of the killers is black . . . we need to understand that it's the person's inner chaos, not his color."

I know it was frustration bordering on desperation that led me to call Svetlana, the astrologer whose column appears in The Washington Post, when I heard that she had cast the charts of 12 of the dead and missing Atlanta youngsters.

She told me the 21 children (she used that figure even though at the time the official number was 19) had in common a sharp intelligence and a sort of smart-alecky fearlessness that ultimately did them in. They were lured, she said, by the prospect of "pie-in-the-sky -- not $5 for candy but the promise of a lifetime, something that would change their lives permanently.

She said she sees the killer as perhaps a woman, or a man dressed as a woman, black and benevolent: not a person the victims met by chance. If she could get the children's exact birth hours, she said, she felt certain that she would be able to cast the chart of the killer. But the Atlanta authorities and the FBI, already burned by a Boston psychic, don't want to deal with her.

The measure of my personal frustration is that I'm suggesting that the authorities ought to talk to Svetlana. After all, what is there to lose?