On this side of paradise, doing evil seems to come more naturally–and certainly more easily–than doing good. There are no innocents: we are all children of darkness struggling toward the light.

This is why every religion aims at a radical transformation of the self and provides moral guidelines for becoming holy in God’s sight. Jews rely on the laws of Moses and the wisdom of the Talmud; Christians look to the teachings of Jesus, the guidance of the Holy Spirit and especially the power of transforming grace; Muslims seek transformation through total submission to Allah, guided by the recitation of the Quran and the example of Muhammad. Every religious tradition also attempts an explanation of evil–and, for believers, a means of escape from its grip.

Evil is more than the sum of individual acts and omissions. Like gas, it expands upon release. Corruption breeds corruption until–as the ancient Persians believed–it seems that twin deities govern the universe: one the god of goodness, order and light, the other the god of evil, chaos and the dark. All religions turn to myths in order to account for the origins of evil. In the most ancient faiths, evil in the form of chaos precedes the advent of human beings, who merely continue it by opposing divine order.

Hinduism and Buddhism both have numerous and overlapping myths involving evil gods and demons. But philosophically they rely on the powerful doctrine of karma, which explains the evil in this life as the fruit of past thoughts and actions in every individual’s previous lives. In Buddhism especially, an abundance of bad karma can produce a future rebirth in one of several hells. But for the Buddha, the real evil is existence itself, since all sentient beings are subject to suffering, death and rebirth. In his view, these evils are ultimately rooted in our attachment to the illusion of a permanent self. In short, evil is explained as ignorance, and the only way to escape the endless cycle of death and rebirth is to realize our own emptiness through meditation techniques aimed at destroying the illusory sense of self.

Every monotheism, on the other hand, must struggle with a metaphysical conundrum: how to explain the existence of evil in a world governed by a single creator God who–by the definition of the Hebrew prophets–is both all-powerful and all-good. (Though the Lord is a complicated figure: in the Book of Job, for example, God emerges as an arbitrary and incomprehensible deity even to the righteous.) Judaism, Christianity and Islam all look to the myth of Eden, where evil enters the world through the disobedience of Adam and Eve, humanity’s primal parents. From this moral failure, natural evils like suffering and death become our common lot, as does the propensity to do evil rather than good. In each of these religions, traditions have added second thoughts and refined reflections. In Jewish mysticism, the Lord withdraws from the world he has created, leaving it up to humankind to repair the breach. In Islam, evil arises whenever the relativity of this world is mistaken for the Absolute that is Allah; avoiding evil means asserting the oneness of God and rejecting every form of idolatry, especially of the self.

For Western culture, the most influential theory has been the Christian theology of Saint Augustine, produced during the dissolution of the Roman Empire. As a young man, Augustine was a Manichaean, believing that the cosmos was torn between equally powerful forces of good and evil. But in his later writings as a Christian, Augustine identified evil as the “privation” of the good–a sort of black hole in the orders of creation that works against the good God wills for all that he has brought into being. From this perspective, evil shadows the good like antimatter to matter; it has no existence in itself, but is immensely powerful in its ability to negate what is and–morally speaking–what ought to be. For Christians, only the gift of God’s own grace can overcome the power of evil in the world and sanctify the life of the believer.

Radical evil–the kind embodied in a Hitler, Stalin or Pol Pot–is personified in the image of Satan; those 20th-century tyrants possess a malevolence that seems beyond the human. A figure condensed from early Jewish and Christian apocalyptic writings, Satan is a pure spirit created good by God who rebelled of his own free will and was cast for all eternity from God’s presence. He remains, for many believers today, a malign personality of immense intelligence and cunning who tempts a fallen humanity but can never force anyone to do evil. In short, evil in its classical Christian formulation is the nothing-ness that God permits, that the Devil wills and promotes, and that human beings freely choose when they sin. Dante captured all these facets in his magnificent depiction of hell. There Lucifer, the once brilliant angel of light, now fallen from heaven through the earth to its core, is stuck fast and in a lake of ice. Where God is warmth and plenitude and light, Satan is cold, dark and trapped in the narrowness of the self.

As the subject of art and poetry, Satan is a window on the changing conceptions of evil. In Milton’s “Paradise Lost” he became a proud rebel: “‘Tis better to reign in Hell than serve in Heaven.” Milton knew what he was doing, observes historian Jeffrey Burton Russell. He made Satan an attractive figure, Russell says, “so we would see ourselves in him.” Later, Romantic poets liked what they saw, transforming Satan into a Promethean figure who challenges the forces of repression. By the 20th century, Satan had largely disappeared from Western consciousness. The self became the only interesting place to be: “Hell,” Jean-Paul Sartre observed, “is other people.”

Evil, though, is not so easily dismissed. Today we experience evil as a random menace, devoid of cosmic significance. We look at people like McVeigh and measure our distance from him. But deep down we fear that it is just a matter of inches. And so across the centuries we pray to be delivered from our enemies and our own shortcomings–to be delivered, ultimately, from evil.