Jerusalem -- For 400 years, Roman soldiers and laborers gathered in underground sanctums to practice secret rituals before a stone carving of a god stabbing a bull to death.
The god was Mithras, the center of a religion that rivaled the new faith of Christianity begun about the same time by a Jewish carpenter, Jesus.
Who was Mithras? What allure did this young god with a peaked cap and a bloody dagger have for tens of thousands of followers throughout Europe and the Mideast?
These mysteries of Mithraism perplexed archaeologists with each new discovery of the more than 400 underground temples stretching from Scotland to Israel.
A recent theory offered by a U.S. scholar says that the answer is in the stars. The theory portrays the ancients as revering the movements of the universe and preoccupied with astrology. Such beliefs made the rapid embrace of Christianity all the more dramatic.
"It was a time of profound insecurity," said Dr. David Ulansey, a visiting professor of Religious Studies at the University of California at Berkeley. "The Mediterranean culture was undergoing a very dramatic shift. Within just a couple hundred years, it went through a dramatic transformation to Christianity."
Mithraism was a secretive cult that was first spread by Mediterranean pirates and seemed to thrive in the outskirts of the Roman Empire among legionnaire soldiers.
Its ceremonies, from what archaeologists can surmise, were dark and fearful. They were held in long, narrow underground temples. The worshipers entered the windowless sanctums through double doors or curved passages, to a dark place surrounded on both sides by about two dozen men.
Only men were permitted in the cult. Their initiation rite required a fervent oath of secrecy, and possibly a trial or a test. One surviving painting seems to portray the initiation ceremony: a naked initiate kneels blindfolded and plaintive as a priest threatens his life with a spear.
There are no written records, no holy books. To keep the secrecy, all of the beliefs of the cult were passed only orally.
Around the initiate in the temple would have been fearsome images: men with horrid masks that flickered in the torchlight, statues of lesser gods, including a lion-headed figure wrapped by a long snake.
"It would have been scary and frightening" to the initiate, said Dr. Israel Roll, professor of classical archaeology at Tel Aviv University. "The more frightened he is, the more profound his belief in salvation."
At the altar of every temple was a stone carving or painting. Invariably, it depicted the capped Mithras -- surrounded by a snake, a scorpion, a dog and a raven -- plunging a dagger into the neck of a bellowing bull.
For years, Mithras was thought to be a Persian god of the same name. Under the shah, the government of modern Persia, Iran, sponsored research about the religion.
But the Persian was an old fellow, not the virile god who killed bulls found in the Roman temples. Scholars eventually concluded Mithraism had little to do with the Persian god.
So then, who was the Roman Mithras?
Dr. Ulansey argues in a recent article in the Biblical Archaeology Review that Mithras was a god invented to explain a gradual XTC shift in the universe noticed by the ancients.
An amateur astronomer, Dr. Ulansey said he looked at a picture of the bull-slaying, and realized suddenly that all the figures in the icon are represented by constellations.
"Everything falls into place," he said. "When that key [of astrology] is used . . . it becomes an open book."
The ancients were acutely attuned to the zodiac sky, Dr. Ulansey concluded. To them, the heavens revolved around the Earth. But about 128 B.C., they discovered a flaw in this steady routine: The spring equinox was gradually occurring at a different time.
Until about 2000 B.C., this equinox -- the first day of spring -- occurred when the constellation Taurus was on the sunrise. But at the time of Jesus Christ, it occurred during the constellation Aries.
(The spring equinox now falls in Pisces, and in about 200 years it will shift to the constellation of Aquarius -- thus the dawning of the Age of Aquarius.) This gradual shift, over a period of nearly 26,000 years for the entire procession of the Zodiac, is now known to be the result of a wobble in the Earth's axis.
But the ancients concluded it could only have been done by a god strong enough to pull or push the heavens, according to Dr. Ulansey.
Mithras was that god. His slaying of the bull represented the ever-so-slow passing of the astrological Age of Taurus the Bull. The picture found in all the temples, Dr. Ulansey argued, is actually an astronomical star map of that change.
The explanation was crucial to the Romans. At the time, death was seen as the start of a journey through the heavens. Only with a proper understanding of the way -- or help from the right god -- could one traverse to a final paradise beyond the stars, they believed.
"I think I have solved the mysteries," said Dr. Ulansey, whose book "The Origins of the Mithraic Mysteries" has been published in paperback and won high reviews.
Not all researchers agree.
"Some of the new generation of scholars overemphasize astrology," said Dr. Roll. "They try to make Mithraism into an astrological religion. It certainly was not. There is no evidence."
Dr. Roll sees the slaying of the bull as a picture of a common act of worship of the time.
"The killing of the bull in the Roman world was an act of sacrifice. It purifies and opens the way for contact with the deities," he said.
Mithraism died out as Christianity -- which required allegiance to only one God -- swept through the Roman Empire in the fourth and fifth centuries.
"The power was shifted," said Dr. Ulansey. "Until then power was believed to be in the cosmic spheres. Christianity went a dramatic step further: It turned toward the earthly, human life, where such things as sin and guilt were much more important than cosmic power."