Could it be Muhammad was not a single individual but a composite of several people and the Koran more of a political manifesto made up of warmed-over older religious tracts?
"I think that it is certain there was nobody who was like the prophet of Islam we read about in the hadiths [the collective body of traditions relating to Muhammad and the Koran]," said Spencer, who wrote the recently published "Did Muhammad Exist?"
"There is no way to tell who or what was really done. Muhammad was supposed to have lived and united Arabia but there's no historical evidence," he told UPI in a telephone interview.
The book highlights anomalies in the historical record kept by non-Muslims of the 7th and 8th centuries.
Chief among Spencer's concerns are the contradictions about the Koran's compilation, the relative importance of Muhammad's tribe, the Quraysh, and whether Mecca really was the center of trade Islamic tradition makes it out to be.
Spencer is not alone in doubting the canonical account.
The BBC last week broadcast "Islam, the Untold Story," in which historian Tom Holland cast doubt on the traditional story. Holland's examination of whether Islam was handed down whole or evolved over time brought immediate reaction from the Muslim youth organization the Ramadhan Foundation in London, which called the broadcast a mockery.
"There is a desire amongst some people trying to change or discredit Islam, whether it's politicians, commentators or broadcasters like Channel 4. The British Muslim community will not allow Channel 4 to distort our faith and our history," Ramadhan Foundation chief executive Mohammed Shafiq said in a news release, demanding an apology.
Spencer, who wrote an earlier book about the life of Muhammad, "The Truth About Muhammad," said Muslims also have dismissed his work as an attack on Islam and threatened him with violence, although no formal fatwas have been issued.
Spencer suggests Islam could have been founded by Christian heretics who rejected Jesus' divinity and wanted to turn back the clock to a time before Judaism and Christianity became established religions.
There is a story in Jewish lore that says a rabbinical decision denying a proposal to make camel meat kosher is what prompted an angry Muhammad to create a new religion in the early 600s.
"I'd never heard that one," Spencer said. "But there no doubt was antagonism between [early Muslims] and the rabbis. Whatever happened, there were some bitter disputes with Jewish groups. ... It does seem clear there was some attempt [by early Muslims] to identify themselves with Judaism, as an extension of Judaism, an offshoot. When they were rejected by the Jews, they [Muslims] turned against them."
Spencer said there's some indication there was a "general monotheistic understanding" among early Arab converts in the Middle East and North Africa.
Muslim tradition considers Abraham, Isaac, Jacob, Joseph, Moses and Jesus early prophets of Islam whose teachings were hijacked. The tradition says Abraham was neither Jew nor Christian.
Spencer's book indicates much of the Koran is made up of reworked Christian tracts originally written in Syriac, a dialect of Aramaic and the primary literary language in the region at the time. He also said a lack of evidence of a written Koran until decades after Muhammad's death indicates the book itself was put together by Arabic empire-builders who wanted to unite their multinational conquests through religion.
Further, he said, by taking the Syriac writings without diacritical marks and adding diacritical marks to make the words Arabic, "they [Arabic rulers] were intentionally making something they could use ... justifying violence." That, he explains, is also why much of the Koran is incomprehensible -- even in Arabic -- including words that have no definition in Arabic or any other language.
Spencer, a columnist for FrontPage Magazine and director of Jihad Watch at the David Horowitz Freedom Center and whose previous books include "The Politically Incorrect Guide to Islam (and the Crusades)," has studied Islamic theology, law and history for three decades. He says he is bothered by the lack of interest in historic research in the Muslim world.
"There have been brief transformations in Judaism and Christianity as a result of historic investigations," he said. If such examination would lead to "rejection of strict literalism [in Islam], perhaps that would involve rejection of passages [used to justify] violence."
Spencer said "Did Muhammad Exist?" is being translated into Arabic in hopes of getting it distributed in Muslim countries.