Why are we good -- even if we have the power not to be? If we have the power to enslave or kill someone, why don’t we? Who, or what, restrains us?

One answer comes from a new movie, Amazing Grace, telling the story of William Wilberforce (1759-1833), the British activist-politician who ended the slave trade. The film, starring Ioan Gruffudd -- that’s John Griffith if you’re not Welsh -- opens tomorrow; it’s undeniably a message movie.

And what message is that? What was the source of Wilberforce’s inspiration? In one word, it was Christianity -- as delivered to Wilberforce by the impassioned witness of John Newton, the slaver-turned-preacher best known today for the hymn Amazing Grace.

Wilberforce was a lost 20-something, a rich kid who had bought a seat in Parliament, when Newton found him and fired him up on the seemingly hopeless cause of abolishing the slave trade. The evil practice had been profitably entrenched in British life for centuries. As Wilberforce later wrote, abolition was "the grand object of my parliamentary existence;" he hoped to please God by making himself "the instrument of stopping such a course of wickedness and cruelty as never before disgraced a Christian country."

It took Wilberforce more than two decades of unstinting effort, but on March 25, 1807, the Slave Trade Act became law. Slavery, a hideous human institution common in just about every culture throughout history, was on its way to ending in the West, thanks to the influence of Christian morality.

And just in time. Because the 19th century was about to witness an unparalleled expansion in human technical power, as mass production and new inventions gave some men -- first in the West, and then, worldwide -- an extraordinary new power. Specifically, the steamship, the railroad and the repeating rifle, followed soon by the machine gun, made it easier for countries to cross oceans and take over whole continents.

Such power must have seemed almost godlike to its wielders; its use, or misuse, spelled the difference between life and death for billions of people. In some modernism-exporting countries, such as Britain, morality generally restrained technology. To be sure, the British colonized many lands in the 19th and 20th centuries, and sometimes were brutal in doing so, but they never used their new tools to reimpose slavery, let alone practice genocide. Instead, oftentimes they devoted much of their material abundance to providing missionaries, education and political reform.

But other countries, with less of a Wilberforcean legacy of uplift and improvement, showed no misgivings about using newfangled machinery for mass murder. Little Belgium, for example, established a factory approach to agriculture and mining in the Congo that killed millions at the turn of the last century. Later, fascists and communists were both eager fully to systematize savagery, using all the latest technology as part of their vision of morality-free modernization.

That’s the story of, say, Germany under the Nazis or Russia under the Soviets, or China under the Maoists. In each instance, there was simply no adequate restraining tradition to thwart the murderous fantasies of evil leaders and their near-infinite capacities for assembly-line slaughter.

As Winston Churchill told the British people in 1940, if they were not successful in thwarting Hitler, "The whole world ... will sink into the abyss of a new Dark Age made more sinister ... by the lights of perverted science."

Happily, William Wilberforce’s spiritual descendants did not lose World War II nor the cold war. And so, while the worldwide struggle against mass-produced oppression has hardly been won, it is possible nowadays to point to many countries where human rights and human dignity are respected.

And, wherever there is such freedom, Wilberforce’s memory must be revered. His life, which can now be seen on the screen, reminds us that good can be more powerful than the most powerful evil.