When John Heller is praying, he's a southern Baptist. When he's working, he's a scientist, with no time or patience for hypotheses that can't be tested, weighed and measured, proved or disproved. The controversial Shroud of Turin should not have interested him one bit.

Catholics have venerated the shroud for nearly seven centuries--since it suddenly appeared in the possession of a French knight--as the burial cloth of Christ. Most scientists have written it off as religious nonsense.

Heller is a biophysicist with a medical degree. He's taught at the Yale University School of Medicine, and he knew the report he was reading, by a forensic pathologist named Dr. Robert Bucklin, was medically accurate.

But what really raised Heller's eyebrows was the fact that Bucklin, deputy coroner of Los Angeles, was describing the physical evidence of the death of a man whose image appears on the shroud.

The shroud is a strip of old linen, 14 feet long, 3 1/2 feet wide. It is patched, frayed, watermarked, stained. On it appears a faint, life-size image of a man, a man with wounds through his wrists and feet, wounds circling his brow. A man with shoulder-length hair and a long, Semitic face, his back scored by welts.

But Bucklin's report, a thermal expert's study that seemed to indicate that the image on the shroud was not paint, and a set of computer-enhanced photographs of that image, all troubled Heller. "What are these stains?" he kept asking himself. "And how did they get there?"

So, against his better judgment, Heller joined the hunt for scientific answers about the shroud. His stated purpose was the same as that of most of the other 39 scientists on the Shroud of Turin Research Project: to debunk what they were convinced was a forgery, or at least the product of a thoroughly explicable natural process.

"It is in our nature and our training to refuse to accept the mystical as an explanation of an object," Heller explained. "I felt that, given adequate time, adequate instrumentation, adequate research facilities, it was impossible that we would not come up with the answers to two questions: What is the image on the shroud made of and how did it get there? I would have made book on it, 10,000 to one."

He would have lost his bet.

"We failed," Heller said matter-of-factly. "The image was not made by the hand of man, and there is no known mechanism, accidental or otherwise--physical, chemical or biological--which can satisfactorily account for it. After five years and 100,000 to 150,000 man-hours of scientific work, it's still a mystery."

Of course, there is a third question. Is this the actual burial shroud of Jesus Christ?

The scientists didn't address that one, and they don't intend to. "I"I n science," said team member and thermal expert Ray Rogers, "you're "I entitled to any hypothesis you choose, including the one that the shroud was made by elves from the Black Forest. But if you don't have a test to examine that hypothesis, it's not worth anything. We do not have a test for Jesus Christ. So we can't hypothesize or test for that question."

Individually, three members of the team--including the project's founder, John Jackson, Bucklin and photographer Barrie Schwortz--are convinced that the shroud is the authentic burial shroud of Jesus.

Heller isn't sure. "I really don't know," he said. "Even if the Carbon 14 examination which the team hopes to conduct next spring dates the shroud as 2,000 years old, most of us probably still will have to say we just don't know."

Heller's hackles rise when antagonists suggest that he and fellow team members were nothing but a bunch of religious zealots out to prove the authenticity of the shroud.

"This scientific adventure was just that. Science," he snaps. "It had nothing to do with faith. Nobody is saying diddley-squat about authentic."

But their work on the shroud did alter the religious convictions of at least some of the scientists. One of the six self-proclaimed agnostics on the team remarked to Heller, "I'm still agnostic, but I have my antennae up now."

Another went even further. "Science was my God," Tom D'Mulhalla, a nuclear physicist, told Heller. "Now I get down on my knees."

When Heller finished his research, he sat down and wrote a most readable account of the five years of science that went into the Shroud of Turin Research Project. Called simply "Report on the Shroud of Turin," the book was published recently by Houghton Mifflin Co.

It reads more like an adventure novel than a scientific tome. Heller manages to bring to life the personalities of the diverse lot of scientists who came together to apply their expertise to the shroud. Physicists, chemists, botanists, biologists, pathologists, endocrinologists, bacteriologists, computer scientists, radiographers, textile experts, mathematicians--on Heller's pages they collaborate and collide, wrestle with the inexplicable and risk their professional reputations in a thoroughly human way.

It's hard not to chuckle when Heller tells of John Jackson's reaction to the three-dimensional image of Jesus that appeared when he fed a photograph of the shroud into a computerized image analyzer, the sophisticated equipment used to produce the space program's pictures of the planets.

"Yabba-dabba-doo!" the nuclear physicist shouted as he drove home from the lab that day.

"Is that the top of your euphoric vocabulary?" Heller asked him. "Yes," Jackson said. "That's the biggest and the best."

And it's even harder not to grin when Heller tells how the archbishop of Turin got the scientists' equipment released from customs, where it was being held against the posting of a $250,000 bond. "His Eminence guaranteed the bond from the clerical account," Heller writes. "When I later asked what exactly that meant, I was told in simple language that he had hocked the cathedral."

The scientific equipment that almost didn't make it to the team's five-day, hands-on examination of the shroud in 1978 was valued at $2.5 million. All of it was donated or lent to the project.

Approximately $600,000 in cash was raised. Team members donated their time, paid their own way to meetings, even used a windfall from playing the commodities market to help finance their hotel bill in Turin.

Working on the shroud cost some team members more than time and money. One, a professor, accepted a new job, sold his home, moved his family, bought a new home, only to be told to drop his research on the shroud or kiss his new job goodbye.

He quit his new position and continued his work on the shroud. HH eller himself faced the embarrassment of an accusation that he was using pubH lic funds to pursue what he says the attorney general of Connecticut called "this hobby of mine." The attorney general even got an injunction against Heller, which was vacated by an angry judge when Heller made the facts known in court.

Heller and the rest of the Shroud of Turin Research Project team are convinced that the shroud is not a painting or any other product of the hand of man. They are sure that the stains on it are human blood.

Walter McCrone is sure they're wrong.

McCrone is an analytical chemist who runs a commercial lab in Chicago. Not a member of the research team, McCrone borrowed the microscope slides the team brought back from Turin, tested them and pronounced the shroud a fake.

The "blood" is from oxide, McCrone said. The image, a painting, made of red ocher--an artist's pigment--and gelatin.

"We bent over backwards to try to find what he said he'd found," Heller said. "Sufficient was not good enough. We made the tests exhaustive."

After more than 1,000 separate experiments, "examining every particle we could find and testing it chemically, we could not corroborate any of his observations," Heller said.

"When with 100 percent certainty, we make a categorical statement that blood is present, believe it!" the biophysicist went on to say. "I have tested for the presence of blood in capital cases for the prosecution and defense. If I had found blood on two samples and one turned out to be Swiss cheese and the other a moon rock, I should, with confidence, tell the authorities to look for a gory dairyman or a wounded astronaut."