A BORN-AGAIN "Mother Goose"? Who needs it? Marjorie Ainsborough Decker says we all do.

So far, half a million customers have agreed. And when the annual Yule blitz subsides at the bookshops and the cash-register smoke clears, the odds are good that this year's number one juvenile title will be one you can't find in most stores: "The Christian Mother Goose."

Written and illustrated by Marjorie Decker, 58 ("and I'm never going to grow old"), of Grand Junction, Colo., marketed by her son Kevin, 26 ("and I'm never going to grow up") and published by the family's Decker Press, "TCMG" now boasts 586,000 copies in print at $10.95 each. The Deckers claim it's the nation's best-selling hardcover children's book. They're probably right.

"Religious books of a fundamentalist nature have just absolutely phenomenal sales," says Robert Hale, associate executive director of the American Booksellers Association. "They never appear on the best-seller lists," he says, because they are not sold in the trade bookstores, which determine the exalted rosters of Publishers Weekly or The New York Times. "But they sometimes outsell -- by hundreds of thousands -- books that are on the lists."

It was an idea whose time had come. Marjorie Decker took a hard look at the nursery rhymes that have lulled English-speaking tots into salubrious slumber for two centuries. And what she found was an infidel horrorscape of blind amputee rodents, blackbird-infested pies and pipers' sons committing grand theft/pig. Not exactly God's little acre. So she set out to purge the offending passages, recast the immemorial verses as Christian homilies and create a sort of gumdrop paradise composed of equal parts Hanna-Barbera, Jerry Falwell and Lord Baden-Powell.

* Remember the old woman who lived in a shoe? With the single-parent household in need of a visit from Planned Parenthood? Here's her draconian solution to the twin heartbreaks of brat-glut and substandard housing: She gave them some broth, Without any bread, And spanked them all soundly$ (KEY OFF)(KEYWORD)nd sent them to bed.

Too grim for Decker. Her old woman . . . had so many children, And loved them all too. She said, "Thank you, Lord Jesus, For sending them bread," Then kissed them all gladly And sent them to bed.

* No less inspiring is the resurrection of H. Dumpty. In the original, the ovoid gent's condition is terminal. But Decker heals the Dumptian rupture through divine intervention: Humpty Dumpty shouted, "Amen! God can put me together again."

* Little Bo Peep, formerly known to millions as an inept sheep rancher, still misplaces her livestock in the Decker version, But Jesus knows And can bring them home, Wagging their tails behind them.

* Similarly, Old Mother Hubbard's bare-cupboard and pooch-chow problems are miraculously relieved when, after prayer, the Almighty provides "bones in a sack." (Not exactly the Red Sea parting, but what do you want for $10.95?)

"My childhood was very rich in nursery rhymes," says the author, who long ago left her native Liverpool for America. She had always been "a serious lover and student of the word of God." But it was not until 1975 that she had The Revisionary Vision. It was a normal night at home: Marjorie Decker was sitting in her bathtub, telling stories to her four boys via walkie-talkie. (She had always thought and composed best in a warm soak, she says, yet the nippers demanded tales. The aquatic broadcast permitted both.) Near her tea tray was a Bible and a copy of "Mother Goose." "It was one of those flashes of inspiration," she says. "I started paraphrasing right there," achieving such rhapsodic heights as this: "Fee, fi, fo, fum,/I smell cookies/That smell yum-yum."

By 1978 she was finished and very excited. She was also rejected by "three major Christian publishers." But the Deckers' faith was as a truckload of mustard seed: They sold the family trophy-making business and published "TCMG" themselves. And lo, the mountain moved: After debuting at the 1979 Christian Booksellers Convention, the book started selling like holy hotcakes.

The gospelized "Goose" is "a phenomenal success," says John Bass, executive vice president of the Christian Booksellers Association. "It's uncanny how it dominates the marketplace," often topping the Christian best-seller list in the CBA's magazine, Bookstore Journal. And it's a huge marketplace: Since the evangelism boom of the mid- to late '70s, the Christian book biz is now a $1-billion-a-year industry, shipping more than 3,000 titles annually to specialty stores and clubs. In that climate, Bass says, "TCMG" was "something the market was ready for."

It had better be: The Deckers are doing for kiddie devotion what Shakey's did for the pizza. Beyond the first book, there's a "TCMG Treasury" of more rhymes (also $10.95), with a third volume due out next summer. There's an LP album (from Word Records, ABC's religious wing), a 94-minute TV special for Christian stations based on Marjorie Decker's own characters (Charlie Cricket, Grandpa Mole, et al.), and the official CMG hot-air balloon for promotions ("a poetic symbol," says pilot Kevin Decker, "of who we are, what we do"). And coming soon, he says -- "America's first line of inspirational toys." Since "there's never been a Smurf for the inspirational market," CMG's plush poppets will fill the piety gap. "We have a hunch that faith is going to be consistently -- and therefore commercially -- a force in American life," he says. Hence the Deckers' ultimate "20-year dream": a Mother Goose Land family theme park.

Why not? In pursuit of their slogan, "Take another gander," Decker has girdled the globe with sales. Marjorie Decker tells the story of an Italian 4-year-old, hideously mangled by a dog bite, who reconciled himself to his disfigurement by reciting the Dumpty credo: "God can put me together again." In Singapore, she has donated 1,000 copies to be read by dying children. And the domestic market is surprisingly broad. "Correspondence tells us," Kevin Decker says, "that Paul Harvey keeps a copy tucked in his nightstand" -- where the Ezekiel of the airwaves may ponder these narrative innovations:

Ms. Muffet, the tuffet-toting picnicker terrorized by a spider in the original, is now a sort of entomological evangelist. Her speech thanking Jesus for feeding the puppies and bunnies and so forth overwhelms the heathen arachnid, who finally bends all eight knees in prayer. Jack and Jill still climb their hill. But instead of a skull fracture, they get a lecture about asking Jesus to give them "living water." The quondam blind mice have become "three kind mice," their imperiled appendages intact. Even "Hickory Dickory Dock" is not exempt: "The clock struck ten,/The mouse said, 'Amen.' " Little Jack Horner now sits in the corner reading his Bible; and "Peter Piper picked a peck, etc." has become "Peter Postle picked a pack/Of praising prophets."

Numerous educators, psychologists and even religious journals have ridiculed Decker's sugared couplets; and child specialists of the Bruno Bettelheim persuasion argue that an early exposure to grim fantasy is essential to a child's development. But Marjorie Decker believes that parents accept only "the more glaringly violent rhymes" because of "the weight and power of tradition." (Mom Goose probably originated in 17th-century France. The first English version, published in the 1760s, was a compilation of 50 familiar British folk rhymes -- many of them covert political satires -- along with several of Shakespeare's songs.) Kevin Decker adds: "We're not trying to eliminate conflict or drama--only to provide a Christian resolution to conflict."

Besides, half a million customers are happy. And so is the bard of Grand Junction. "Any man can put his foot on the soil of America," Marjorie Decker says, "and hope to fulfill his grandest dreams." CAPTION: Picture, author Marjorie Ainsborough Decker with a copy of "The Christian Mother Goose Book," Copyright (c) 1982, Decker Press Inc.