By John Blake, CNN

Editor’s note: The CNN documentary 'Slavery's Last Stronghold' airs on CNN International TV March 29, 30, 31 and April 22. Check local listings for times.

(CNN) - Which revered religious figure - Moses, Jesus, or the Prophet Mohammad - spoke out boldly and unambiguously against slavery?

Answer: None of them.

One of these men owned slaves, another created laws to regulate - but not ban - slavery. The third’s chief spokesman even ordered slaves to obey their masters, religious scholars say.

Most modern people of faith see slavery as a great evil. Though the three great Western religions – Judaism, Christianity and Islam – disagree on many matters, most of their contemporary followers condemn slavery.

Yet there was a time when Jews, Christians and Muslims routinely cited the words and deeds of their founders to justify human bondage, scholars say.

At times, religion was deployed more to promote the spread of slavery than to prevent it.

Read about present-day slavery in Mauritania

“The lesson in all this is we need historical humility,” says Daniel C. Peterson, author of “Muhammad, Prophet of God.” “It’s stunning for us to look back now and say, how can people face themselves in the mirror after doing what they did, but they did.”

But what did the founders of the three great Western religions do? Did they have slaves and did they condemn the practice? Or were they, at least on this issue, squarely men of their times?

The answers to these questions are as murky and contradictory as history itself.

What’s a slave?

Part of the problem is historical context. Most contemporary people think of slaves as people condemned to a lifetime of bondage, working on plantations and being whipped like oxen.

That kind of slavery did exist during the lives of Moses, Jesus and the Prophet Mohammad. Many slaves were prisoners of war; concubines, gladiators, laborers in salt mines. They could be killed, raped and discarded at any moment.

Yet there were layers of slavery in the ancient world. Many slaves would be seen today as indentured servants, or people trying to pay off debts; royal bodyguards and entrepreneurs, historians say.

CNN’s Belief Blog: The faith angles behind the biggest stories

Sometimes the slaves became masters. In medieval Egypt, Muslim rulers trained and educated slaves to be their bodyguards. One group of slaves grew so powerful that they overthrew the rulers of Egypt and established their own dynasty, says Ali Asani, a professor of Indo-Muslim and Islamic Languages and Culture at Harvard University.

“Slavery meant different things in different cultures,” Asani says. “There wasn’t always this sense of powerlessness and oppression. In certain forms, it became an access to power.”

In other forms, it became access to freedom, says John Dominic Crossan, one of world’s leading scholars on the life and times of Jesus.

That was the case in the world of Jesus. The Roman Empire was the dominant power of Jesus’ day, and it survived on the backs of millions of slaves. Yet there was only one mass slave revolt against Rome, which was led by Spartacus, a gladiatorial slave, Crossan says.

The reason there were so few massive slave rebellions against Rome was because some of its slaves had avenues for advancement, dim though they may seem to modern sensibilities.

Slaves could buy their freedom. They ran businesses for their masters or tutored their children. Greek slaves, in particular, were often valued because of their education and culture, he says.

Follow the CNN Belief Blog on Twitter

Roman slavery was cruel and capricious, but not all Romans saw slaves as subhuman.

“One of the most extraordinary aspects of Roman slavery,” says Crossan, author of “The Power of Parable: How Fiction by Jesus became Fiction about Jesus,” was that the Romans ended up with a huge number of slaves who were smarter than their masters.”

The uncomfortable historical record

It’s been said that great religious figures transcend history. They rise above the peculiar customs of their day to show a new path forward.

It’s a matter of debate if Moses, Jesus and the Prophet Mohammad did that with slavery. All three seemed to either ignore or tolerate some forms of slavery, some scholars say.

The parables of Jesus, for example, were full of references to slaves. Terms like “servants” or “stewards” are what we would call slaves today. Yet Jesus doesn’t seem to make any moral judgments about slavery in his parables, Crossan says.

The subject may have been irrelevant to him or his audience, says Crossan, the Jesus scholar. Jesus didn’t own any slaves. Neither did his disciples or the crowds Jesus addressed. They were all too poor and lived under desperate economic circumstances.

“It may well be that the people he talked to were small farmers who would not have the luxury of slaves,” Crossan says. “He [Jesus] doesn’t say anything for or against it.”

Still, Crossan says that he believes that Jesus would have opposed slavery, given the nature of his teachings. Scholars aren’t so certain about Jesus’ most influential disciple, the Apostle Paul.

The man whose writings make up most of the New Testament had to deal with slavery. As Christianity spread through the Roman Empire, many slaves joined the church.

At various parts of the New Testament, Paul seems to accept slavery. He tells slaves to obey their masters. At other times, Paul seems to challenge the morality of slavery. In one New Testament letter, Paul intercedes on behalf of a runaway slave and chides the master for calling himself a Christian and holding a slave.

Crossan, along with some other biblical scholars, says there are actually two versions of Paul in the New Testament: the authentic, “radical” Paul who opposed slavery and a “Pseudo-Paul” inserted into the texts by early church leaders who were afraid of antagonizing Rome.

“It’s one thing to say that Jesus is Lord,” Crossan says. “Now if you’re saying a Christian can’t have slaves, then something must be wrong with slaves. So now you’re attacking the Roman system, which is a slave economy.”

Jesus’ apparent silence on slavery and Paul’s ambiguous statements on the issue had dreadful historical consequences. It helped ensure that slavery would survive well into the 19th century in the U.S., some scholars say.

American Christians who owned slaves had a simple but powerful defense in the run-up to the Civil War. The Old and New Testament sanctioned slavery and, since the Bible is infallible, slavery is part of God’s order, says Mark Noll, author “The Civil War as a Theological Crisis.”

“The defenders of slavery said Jesus condemned quite a few things that were standard in the Old Testament,” Noll says. “He condemned polygamy, violence, easy divorce, but he never condemned slavery.”

Let my people go, but keep the others

Neither did Moses, the founder of Judaism, say other scholars.

There’s no record of Moses owning slaves, but the Mosaic laws permitted and regulated slavery, says Peterson, the author of “Muhammad, Prophet of God” and a religious scholar at Brigham Young University in Utah.

Still, under Mosaic law, a master was encouraged to free slaves and forgive debts after a certain period of time that was called the year of jubilee, Peterson says.

“They were not trying to create a permanent underclass of slaves that went from parents to child and child and grandchildren,” Peterson says of the ancient Israelites.

But how could ancient Israelites sanction any form of slavery given their exodus from Egyptian captivity? Didn’t their God explicitly condemn slavery when he ordered Moses to tell Pharaoh to “let my people go?”

The text is not clear on that question, says Brannon Wheeler, a religious scholar.

He says the Exodus stories suggest that the God of Israel was angry at Pharaoh not for enslaving a group of people, but for unjustly enslaving the “Chosen People” - the people God had promised to give their own homeland.

“In order to make that promise stick, He [God] has to get them out of Egypt,” says Wheeler, director of the Center for Middle East and Islamic Studies at the United States Naval Academy in Maryland.

“It’s not like He [God] says slavery is bad and I want to abolish it.”

The Prophet Mohammad never explicitly condemned slavery, and actually owned slaves, some scholars say.

Yet he recognized the humanity of slaves, teaching followers that freeing slaves was an act of piety. He allowed slaves to buy their freedom and demanded that they should be treated with love and respect, says Asani, author of “Celebrating Muhammad: Images of the Prophet in Popular Muslim Poetry.”

“He himself did own slaves but he treated them as family,” Asani says. “One called Zayd he treated like an adopted son and one of his wives was a Coptic Christian slave.”

The followers of men like the Prophet Mohammad, though, would take a harsher attitude toward slaves.

By the time of the crusades, Christians and Muslims were enslaving one another by the thousands. They cited their faith as justification, says Robert C. Davis, author of “Holy War and Human Bondage.”

“Religion was the defining principle of slavery—this person is another faith and can be enslaved,” Davis says.

Some church leaders preached that enslaving others was an act of evangelism, Davis says.

“One pope said that the justification for slavery was that it was important for spreading the faith,” Davis says. “Once they were enslaved, they would more readily take to Christianity.”

Those kinds of actions may now seem barbaric, but the texts and stories that were used to justify slavery still exist in the sacred texts of Judaism, Christianity and Islam.

Few, though, would quote those scriptures today, and many don’t even know they exist.

“We shouldn’t be surprised,” says Jonathan Brockopp, a religion professor at Pennsylvania State University. “Religions redefine themselves and people draw on different stories and underplay other stories. This happens constantly.”

It happened with slavery, and, who knows, perhaps it’s happening again in our time. There may be a religious practice accepted today that future generations will look upon and ask the same question we ask about people who enslaved others in the name of God:

How could they?