Great evil, like great goodness, is a mystery. Both beggar the imagination and exhaust our explanations, leaving us simply in awe of what a human being can do.

An ageing mother who nurses her chronically ill and helpless daughter at home for 20 years displays a heroic goodness that outstrips that of the soldier who, in the heat of battle, rescues a stricken comrade under fire. We can understand the soldier and imagine doing the same, but could we commit ourselves, indefinitely to the monotonous and often thankless tasks of bodily care and emotional encouragement that such nursing requires? Could we persevere, day after day, year after year? Such self-sacrifice is surely beyond the call of love as well as duty.

Even more so the act of the Polish priest, Maximilian Kolbe, who volunteered to take the place of another prisoner selected among others to be shut in the starvation bunker at Auschwitz in payment for an escape attempt. To freely embrace such a terrible death for the sake of another -- really, for the sake of all the others, since he wanted to minister to them in that hell -- belongs at the dizzying heights of goodness, or an extreme of madness. The Catholic Church has decided it is the former and canonised Kolbe. Who dares to think they could follow his example?

And yet, one can more easily imagine being that mother, that soldier, even that priest, than being Josef Fritzl, the Austrian father who locked up his daughter underground for 24 years, made her his sex slave and did what he liked with the children who were born. Why? How could he? Could we? Never! Even in a world grown accustomed to stories of perversion, the story of the Fritzl family has caused universal shock and revulsion.

It takes us nearer to what Joseph Conrad called the “heart of darkness” than we have been for a long time. There have been stories of heart-stopping inhumanity, stories of sickening depravity in recent years, but nothing to beat this one. Marc Dutroux, the Belgian sex offender, imprisoned, raped and traded several young girls, causing the deaths of at least two, but at least they were not his own daughters. Armin Miewes, the Rottenburg cannibal, killed and ate another man, but at least the victim was an adult who “volunteered” for the experiment. Then there was that other Austrian teenager, Natascha Kampusch, held captive in a basement as a sex slave for eight years, but even in her case the circumstances and results were not nearly so extreme.

Perhaps the closest approximation to Fritzl’s crime are those cases, which occur regularly in every country, of children physically and even sexually abused, sometimes for many years, by their parents (often a live-in boyfriend), relatives or other supposed caregiver. Often these children go to their deaths having known nothing but rejection, fear and pain. However, the degree of calculation behind Fritzl’s cruel oppression of his daughter, its duration and extension to children born to a life of unimaginable confinement -- these and other features put the case in a class of its own.

At the same time we have to admit that it is not a totally isolated case of monstrous wickedness. It is less than 70 years since the Nazi doctors performed their horrendous live experiments on concentration camp prisoners. Only a century or two before that “civilised” slave traders and plantation owners were treating African captives with unbelievable cruelty. Going back through the centuries one confronts a long tradition of torture as a political tool -- one that is still, scandalously, with us today. Man’s capacity for hardness of heart seems fathomless.

Of course, one must try to understand the Fritzl case if for no other reason than preventing another, and suggestions have not been lacking. Austria’s Nazi past is supposed to have left its mark on him; Natascha Kampusch herself has suggested as much. One commentator has insinuated that there is something inherently suspect about any “respectable family man” since this was the posture Josef Fritzl adopted. Fritzl’s lawyer says he is insane.

British columnist Dominic Lawson pours cold water on the whole idea of explaining the man’s behaviour, saying it is “simply inexplicable”. And it is -- if we confine ourselves to merely human explanations. History, culture, psychiatry, even that fundamental reason-for-everything, evolution, fail to answer the question why a father would do what Josef Fritzl has done to his daughter. (What, by the way, would be the evolutionary advantage here?) But agnosticism is too easy.

No, the Fritzl case demands that we try to penetrate the mystery of evil, and that leads where 21st century secularists do not wish to go: to the non-human wellsprings, the supernatural force that itself can hold a man (or woman) captive once they surrender to it. I mean, of course, the Devil, the fallen angel who wishes to avenge himself against God by dragging his human creatures one by one into hell.

The remedy against this force is the grace of God, which is stronger than the force of evil but cannot save a person if he does not freely surrender to it. At some point in his life of abuse Josef Fritzl had the freedom to continue or to stop what he knew was gravely wrong. Indeed, at any point he could have stopped, but the longer he persisted the harder it became to call a halt. It was not just his own weakness leading him on but the demon that had taken up residence where his conscience should have been.

The Devil -- or an impenetrable yet ever-threatening mystery. Like it or not, those are the alternatives. A supernatural explanation for evil, or no explanation. Cry “Rubbish!” at the idea of God, grace, the Devil and his power and we are left with no explanation. No explanation -- or inadequate explanations -- means no prevention. It means more of the same, and worse. As long as religion remains excluded from our social calculations it will be easier for evil to flourish and more difficult to achieve the good we want. Who can look at the state of our Western societies and maintain that they are winning the war against sexual abuse and all forms of exploitation?

One final word about Josef Fritzl. Even here, the powers of evil do not have the last word. It would have been logical for someone who only cared about saving face -- the phoney “respectable family man” -- or someone totally self-obsessed to have ended the whole dreadful saga by killing himself, his daughter and the three children with her in that underground tomb. Instead, he finally set them free. Even then, it seems, he made up some story to disguise his crime and now he hopes to pass as insane, but the fact remains that he did one right thing at the end. No-one is beyond redemption, but it takes more than natural faith to believe it in his case.

Carolyn Moynihan is deputy editor of MercatorNet.