EDITOR'S NOTE — This is another story in an occasional series examining the fault lines within Islam between the forces of moderation and extremism. The consequences of this fight that shook America on Sept. 11, 2001, reverberate today, and the struggle for Islam may be the defining conflict of our era.

LOS ANGELES — UCLA law professor Khaled Abou El Fadl has a scholarly manner and speaks in soft tones. But listen as he tells his story.

A Kuwaiti native, he was fascinated by militant Islam as a young man, then evolved into a moderate champion of democracy and suffered arrest and torture in Egypt for his views. Saudi go-betweens failed to buy his silence but long limited his influence by preventing publication of his works in Arabic. He received death threats over anti-terrorist comments following the Sept. 11 attacks.

Now, as Muslim immigrants to America struggle to find their voice, no one is more outspoken than Abou El Fadl — driven by what he sees as a global crisis: the fight between "moderates" and "puritans" to determine who represents authentic Islam.

"Nothing less than the very soul of Islam" is at risk, says the 42-year-old Abou El Fadl, who is calling upon moderates to reverse their declining influence and reclaim bold leadership of the faith.

This is a "transformative moment," he says. In his view, Islam is suffering a schism as dramatic as the 16th century Protestant Reformation that split Christian Europe.

Two main movements claim to perpetuate true Islam, he says. On one side, the professor's fellow moderates uphold centuries of Muslim teaching and the beliefs of an often quiescent Muslim majority.

Their opponents, as he sees it, are puritans — he dislikes the "fundamentalist" and "Islamist" labels — who've won a remarkable following as they've preached religious extremism and, often, carried out acts of reprehensible violence in recent decades.

Eventually, one of these two rivals will achieve near-total commitment from the world's more than 1 billion Muslims and "the power to define Islam" for the indefinite future — including attitudes toward terrorism, he predicts.

Abou El Fadl depicts the contest in his new book "The Great Theft: Wrestling Islam from the Extremists" (HarperSanFrancisco). It's probably the most dramatic manifesto from an American Muslim since the Sept. 11 attacks.

Reaching this point has been a complex, dangerous and sometimes lonely struggle for the author.