Religious differences fuel many of the world's violent conflicts, detractors and supporters of organized faith often lament in unison.

Author Jonathan Kirsch would put a finer point on the charge. He blames the leading monotheistic religions--Judaism, Christianity and Islam--for much of history's bloodshed. The reason, he maintains, is monotheism's traditional claim to exclusive possession of absolute truth.

Too bad Julian the Apostate, the Roman Empire's last pagan emperor, died young in battle, says Kirsch, author of the newly published "God Against the Gods: The History of the War Between Monotheism and Polytheism" (Viking). Had Julian lived longer, he might have succeeded in reinstating classical Greco-Roman polytheism, which was marginalized when Emperor Constantine the Great institutionalized Christianity's ascendancy--and world history might have turned out more benign.

"Julian is one of the great 'what ifs?' of history," said Kirsch, whose earlier books include "The Harlot by the Side of the Road: Forbidden Tales of the Bible" and "King David: The Real Life of the Man Who Ruled Israel."

"Human history is the history of our evolution toward greater individual liberty. I have the nagging feeling that, at least in the West, we might have gotten there faster and in a more direct way had Julian lived."

Polytheism, the belief that there can be more than one god, was the ancient world's dominant religious system. Today it survives chiefly in Hinduism, in tribal traditions, in Afro-Caribbean faiths such as Santeria and Voodoo, and in Wicca and other neo-pagan movements that are growing in North America and Western Europe.

Greco-Roman polytheism reached its philosophical peak in Neo-Platonism, which emphasized ethical behavior and the existence of a unifying transcendent reality.

Polytheism's core value, Kirsch writes, is theological pluralism, a stark contrast to traditional monotheism's penchant for insisting that the "One God" demands theological conformity. And religious freedom, the 54-year-old Kirsch said in a telephone interview, paves the way for public differences of opinion on other topics as well. "The greatest achievement of American democracy is the rigid separation of church and state," said Kirsch, who maintains a full-time practice as an intellectual property attorney in addition to his prolific writing career as an author and book critic. Veering into contemporary issues, Kirsch added, "If you are intellectually honest about separation of church and state, you have to be in favor of gay marriage and removing 'under God' from the Pledge of Allegiance."

In his book, Kirsch--who labels himself a "popularizer" of contemporary Bible scholarship, in which he is largely self-educated--begins the story of monotheism's rise with Akthenaton, the 14th century B.C. Egyptian pharaoh and proto-monotheist. (Kirsch skips the biblical prophets Abraham and Moses, whose historical reality he rejects as unproven.)

Not until the reign of King Josiah, the seventh century B.C. ruler of the Jewish kingdom of Judah, did the biblical Israelites fully elevate their chief god, Yahweh, to the status of the "One God." "Judaism as a faith of strict monotheism can be said to begin with King Josiah," said Kirsch.

Kirsch devotes the greater part of his book to the reigns of Constantine, who embraced Christianity and made it Rome's official faith in the fourth century, and Julian the Apostate, Constantine's nephew who briefly restored polytheism to its traditional place in the Roman pantheon for one last time.

Christian writers emphasize Constantine's faith conversion as the root of his embracing Christianity. Kirsch emphasizes Constantine's political motivations.

In a chapter subtitled "The Invention of the Totalitarian State by the First Christian Ruler of Rome," Kirsch writes that Constantine's "preference for monotheism over polytheism reflected his own ambition to achieve the same absolute power on earth that the Christian god was believed to exercise in heaven."

Likewise, Kirsch continues, Constantine convened the Council of Nicaea in 325--out of which Christian tradition says came the faith's central statement of doctrine, the Nicene Creed--more out of a desire to impose control over an increasingly unwieldy church than out of concern for theological clarity in pursuit of spiritual truths.

Julian--who came to full power in 360, following Constantine's death and after some years of nasty internecine intrigue--was a pagan counterrevolutionary who restored religious legitimacy to classical Greco-Roman polytheism. However, Kirsch emphasizes, Julian did not try to eradicate monotheism as Rome's Christian rulers had sought for polytheism. Julian instead sought to place polytheism and Christianity on equal footing. "That's what's most appealing about polytheism--its openness to accommodating the faiths of others," said Kirsch.

Kirsch may have a sweet spot for polytheism, but he is no Pollyanna. He fully acknowledges that polytheists--including pre-Christian Romans--can be as brutish as fervent monotheists (his preferred term for fanatical fundamentalists). The only difference between violent polytheists and violent monotheists is that the former kill to gain political control and the latter kill to assert theological dominance.

The difference is subtle, said Kirsch, but important. Polytheists sought control over the public sphere alone; monotheists sought control over private thoughts as well.

Kirsch noted that traditional monotheists generally dismiss his writing out of hand as uninformed and anti-faith. Yet he insists that he is a "Jewish monotheist."

"I recite the Sh'ma (Judaism's creedal statement of monotheistic orthodoxy), but I also entertain the idea that there are many ways that people perceive the one, true God. My beliefs are not threatened by dissenting views."

Kirsch recounted a Buddhist aphorism to sum up his religious beliefs: "One moon, many pools. Many pools, one moon." The point, he explained, is that light from a single source can be reflected in many ways.