Ann Landers has quoted it in her syndicated advice column. Millions of Americans and others around the world have bought it as a poster or plaque to hang in their homes. Greeting card companies have sold hundreds of thousands of cards bearing its verse.

"Desiderata," an inspirational prose-poem of gentle language and supposedly 17th century origin, has been heralded as a beacon of light illuminating modern-day life with its ancient wisdom. Offering common-sense advice for enjoying life ("Go placidly amid the noise and the haste . . . Be yourself . . . You are a child of the universe, no less than the trees and the stars," it has become world famous in recent years as a credo for living joyfully and peacefully.

But as the result of a 20-year-old case of mistaken identity, it is bringing anything but joy and peace to the Baltimore church with which it has become associated.

The cause of the distress is a seemingly innocuous credit line accompanying the peom: "Found in Old Saint Paul's Church, Baltimore; Dated 1692."

The date has drawn an endless stream of calls, letters and would-be pilgrims to the church, some pestering the pastor and his aides for a glimpse of an old gravestone or some other ancient marker bearing the famous verse.

In fact, there is no such marker because contrary to what all the posters printed on imitiation parchment would have one believe, contrary to what the old-time script might suggest, "Desiderata" is a product of the 20th century, not the 17th. It was composed by a Terre Haute, Ind., lawyer for an overall manufacturing firm.

"I'm sick of it," said the Rev. Halsey M. Cook, rector of Old St. Paul's. "I've been dealing with it 40 times a week for 15 years. Forty times a week."

"We just wish it was forgotten," agreed Henry Haller, administrative assistant to the Episcopalian congregation in Baltimore's central business district.

How it came to be regarded as the wisdom of the ages, and how Old St. Paul's figured in its own troubles, is a tale that even its true author might appreciate.

The church began getting calls about "Desiderata," Mr. Cook and Haller recall, in the early 1960s. The peom had found a foothold in California, where San Francisco's "flower children" embraced it delightedly as a centuries-old affirmation of their philosophy of love and peace. And from there it spread out, as underground printers, thinking they were dealing with a work in the public domain, started cranking out inexpensive posters.

Momentum picked up even more in the mid '60s. After Adlai E. Stevenson died in 1965, a guest in his home found a copy of "Desiderata near his bedside, with notations indicationg he had been planning to use it on his Christmas cards. The publicity that followed spread the peom's fame - and Old St. Paul's - from basements to board rooms.

But the questions began cropping up. Some of the words in the poem, literary authorities pointed out, were not even in use at the time it was supposedly written. And Old St. Paul's kept denying any claim to it.

Gradually, a Baltimore connection emerged. The rector at Old St. Paul's in the late 1950s, it turned out, had been fond of essays and poems of an inspirational nature. It was often his practice, says the Rev. Frederick Ward Kates, now retired and living in upstate New York, to mimeograph the wirtings he liked in booklet form and place them in pews around the church.,

During Lent one year, Mr. Kates recalled in a telephone interview, he came across "Desiderata" ("probably," he says, "in a barber shop reading a magazine") and decided to use it on the front page of one of his booklets.

And as was also his practice, he made sure that the booklet carried the letterhead "Old St. Paul's Church, Baltimore, A.D. 1692" the year of the church's founding.

Someone who attended Lenten sevices, Mr. Kates theorizes, picked up the booklet, took a liking to "Desiderata," and passed it along to a friend, who passed it along to a friend, and so on. And someone along the way who reprinted it obviously transposed the church's letterhead into the misleading credit line that has been plaguing Old St. Paul's ever since.

Whoever picked it up omitted the name of the author, a relatively obscure Indiana poet by the name of Max Ehrmann.

Ehrmann, born in Terre Haute in 1872, made his living practicing law and business (deputy prosecuting attorney of Terre Haute at one point, credit manager ofr a family-owned overall manufacturing firm for 10 years). But his real love was writing, especially philosophical poems and plays.

He composed "Desiderata" in 1972, he wrote in his journal at the time, out of need to remind himself how he wanted to live his life. (The title is Latin for "things to be desired.")

Like most of his writings, "Desiderata" failed to attract much attention during his lifetime. Three years after his death in 1945, his window, Bertha K. Ehrmann, tried to gain more attention by getting "Desiderata" and some of his other poems published as "The Poems of Max. Ehrmann."

"I think if I like it, I'd feet better about it," complains Mr. Cook. "But I think it's the worst piece of gratuitous advice I've ever read. It's the type of advice every Jewish mother gives her son when he goes off to college - 'Wear your rubbers,' 'Don't pick your nose in public. Every verb in it is an injunctive - 'do,' 'go,' 'smile,' 'you should do this,' 'you should do that.'

"When you're all uptight, I think it's a cheap shot to say, 'Dear girl, go placidly.' When I'm uptight, I can'to go placidly. That's the point.

"It sounds so good. It sounds sweet. But nobody can follow it."

If Cook and his associates are unhappy about "Desiderata's" close association with their churcy, they're not alone. A group of Max Ehrmann's fans back home in Indiana think it's about time their man got the recognition he's due.

"Every time one of us finds 'Desiderata' listed as coming from Old St. Paul's Church, we fire off a letter to whoever did it and tell them about it," says Gene Vaughn, president of Friends of Max Ehrmann.

"Max worked hard on his writing. He used to pay for the printing of his own poems. And here he wasn't even getting any credit."

In the meantime, Ehrmann's current publisher, RObert L. Bell of Melrose. Mass., is doing all he can to set things straight. Bell, who bought the rights to Ehrmann's poems from a nephew, says he spends much of his time "chasing infringers," both in and out of courtrooms.

At stake is not only Max Ehrmann's reputation, but the royalties on a "Desiderata" retail market that Bell estimates runs between $1 million and $2 million yearly.

Not only the credit line needs correction, he notes. One of the first infringers, he says, changed some of the poem's words, and the mistakes have been perpetuated. "Be cheerful" in the original, for expample, often comes out "Be careful." Hardly the spirit Ehrmann intended.

As far as Old St. Paul's Church is concerned, rectification can come none too soon. But Henry Haller is not that confident that it will come at all. Even the appearance of new posters with the proper credit line fails to comfort him.

"Someone will just print up another," he says, shaking his head and chuckling. "And the whole thing will start all over."