Unsurprisingly, it is only after the Enlightenment that atheism became a real intellectual force. The Enlightenment produced a massive shift in the conditions of belief. In the great turn to the subject, in the division between the phenomenal and the noumenal, as Kant famously construed it, even in the rise of historical analysis and modern science, there was a great epistemological shift in Western consciousness, and the result was a new opportunity for the denial of belief in the supernatural in general and the denial of a personal supernatural God specifically. Doubt came to be considered as an intellectual tool, and there arose a culture of doubt and skepticism. In the period from the sixteenth to the eighteenth century, the conditions of belief changed dramatically.

One way to understand what happened is to consider what kind of god was left in the wake of Enlightenment thought. For example, if you consider carefully the philosophy of Immanuel Kant, it is clear that he believed in God. But it is not clear at all that he believed in a supernatural, personal God—and certainly not in a God who intervenes in human history. What was left in the wake of the Enlightenment was no longer a fairly monolithic affirmation of theism, but rather a plethora of movements that also included skeptics and freethinkers, as well as Deists and pantheists.

In the late nineteenth century we finally arrive at the four horsemen of the modern apocalypse—Friedrich Nietzsche, Karl Marx, Charles Darwin, and Sigmund Freud. To mention those four names together is to represent a massive cultural, intellectual, and epistemological shift. Each of these men contributed to human thought in a way that changed the conditions of belief, the intellectual foundations of all thought.

Take Sigmund Freud for example. The unconscious, Freud said, explains more than does the conscious. Indeed, it is the precondition of the conscious. Given that, it is easy to see why Freud would believe that religion is merely an illusion that would eventually pass away. Long before Freud came the publication in 1859 of Charles Darwin’s book The Descent of Man.1 From 1859 until the death of Freud at the beginning of the Second World War, an entire change of thinking had taken place, at least among the intellectual classes. Nietzsche, of course, the most abrupt and abrasive of these thinkers, actually celebrated the death of God. In his book The Gay Science, Nietzsche declared flatly that “God is dead,” which was his way of saying that belief in the Christian God had become unbelievable.2 In his work The Anti-Christ, he went on to write that the worst enemy of human enlightenment and progress is the Christian. He refers to Christianity and to Christians in particular as the “domestic animal, the herd animal, the sick animal—the Christian.”3 He said:

Christianity has taken the side of everything weak, base, failed; it has made an ideal out of whatever contradicts the preservation instincts of a strong life; it has corrupted the reason of even the most spiritual natures by teaching people to see the highest spiritual values as sinful, as deceptive, as temptations. The most pitiful example—the corruption of Pascal, who believed that his reason was corrupted by original sin when the only thing corrupting it was Christianity itself! 4

So Nietzsche declared war on theology:

I wage war on this theologian instinct: I have found traces of it everywhere. Anyone with theologian blood in his veins will approach things with a warped and deceitful attitude. This gives rise to a pathos that calls itself faith: turning a blind eye to yourself once and for all, so you do not have to stomach the sight of incurable mendacity.5

And:

The Christian idea of God—God as a god of the sick, God as spider, God as spirit—is one of the most corrupt conceptions of God the world has ever seen; this may even represent a new low in the declining development of the types of god. God having degenerated into a contradiction of life instead of its transfiguration and eternal yes. God as declared aversion to life, to nature, to the will to life. God as the formula for every slander against “the here and now,” for every lie about the “beyond.” God as the deification of nothingness, the canonization of the will to nothingness! 6

In one famous essay he ends with these words: “And all the while, this pathetic God of Christian monotonotheism instead, acting as if it had any right to exist, like an ultimatum and maximum of god-creating energy, of the human creator sprititus! this hybrid creature of ruin, made from nullity, concept, and contradiction, who sanctions all the instincts of decadence, all the cowardices and exhaustions of the soul!”7 Nietzsche declares the necessity of God’s death in order for humans to find liberation in this new intellectual age. He also suggested that Christianity itself was a vile and pathetic faith that produced vile and pathetic creatures. Any creature, he said, who would need belief in God—any creature who would need prayer, any creature who would exercise faith—is a creature whose will is so corrupted by the virus of Christianity that it cannot contribute to society and the building of a strong people.8