The Big Ideas series has for several months now explored the meaning of a number of familiar intellectual phrases, among them Marshall McLuhan's "the medium is the message", Hannah Arendt's "the banality of evil" and Adam Smith's "invisible hand". But none of these feels quite as big an idea as Friedrich Nietzsche's "God is dead". After centuries of Christianity, a new dawn is being announced. And the language Nietzsche uses in his famous passage from The Gay Science reflects the enormity of his discovery: "How could we drink up the sea? Who gave us the sponge to wipe away the entire horizon?" Nothing again will ever be the same.

But what is his discovery? It isn't a eureka moment in which Nietzsche comes to understand that God does not exist. Indeed, he is not all that interested in the question of God's existence. The Guardian cartoonist Martin Rowson recently told me that he would be an atheist even if God walked into the restaurant. Similarly for Nietzsche, it's not a question of evidence or the lack of it.

He is in a completely different place to the new atheist brigade of Richard Dawkins and AC Grayling. If God walked into the room, Nietzsche would stab him – for his "God is dead" revelation is that humanity can only become free if it rejects the idea of the divine. Christianity is not a mistake. It is wickedness dressed up as virtue.

Nietzsche himself was raised in an overly pious religious household. And on the death of his father, who was the local pastor, Nietzsche was brought up to fill his father's shoes. In his first year away from home he wrote some nauseatingly sentimental Christian poetry and won the university preaching prize.

But all this weight of expectation was profoundly claustrophobic and so it was almost inevitable that rejecting God came as a great release. Indeed, such was the enormous freedom that Nietzsche felt in throwing off his Christian upbringing that he came to describe it in terms of salvation. With the most extraordinary rhetorical daring, he borrowed the language of Christianity to articulate the liberation he discovered in this new-found lack of faith. Which is why one of European culture's most dedicated atheists can sound so religious. And why the death of God story feels so much like a biblical parable.

Nietzsche's case against Christianity was that it kept people down; that it smothered them with morality and self-loathing. His ideal human is one who is free to express himself (yes, he's sexist), like a great artist or a Viking warrior. Morality is for the little people. It's the way the weak manipulate the strong. The people Nietzsche most admired and aspired to be like were those who were able to reinvent themselves through some tremendous act of will.

I have never seen anything to admire in Nietzsche's view of morality or immorality. He was badly interpreted by the Nazis. But his ethics, if one can call them that, are founded on the admiration of power as the ultimate form of abundant creativity. His hatred of Christianity comes mostly from his hatred of renunciation and the promotion of selflessness. Jesus was a genius for having the imaginative power to reinvent Judaism but a dangerous idiot for basing this reinvention on the idea that there is virtue to be had in weakness. The weak, Nietzsche insists, are nasty and cruel. They take out their frustration on those who have the power of genuine self-expression.

It may seem perverse but it was Nietzsche who was partly responsible for my own conversion to Christianity. As a philosophy student in the 1980s, I had served my time with the analytic tradition and its logic-chopping ways. Like many students, I was expecting something more from philosophy than an ability to break down "the cat sat on the mat" into its semantic parts or wading through dreary and unconvincing proofs about the existence of God. I wanted the excitement of big ideas. Marx did it for a while. But my own public school version of revolutionary communism was inevitably a brittle thing, despite its evangelical fervour.

As radical socialism collapsed around my ears, Nietzsche invaded my consciousness with a whole range of new and exciting questions. I took the anti-God line entirely for granted. As a good communist, atheism had always been my unexamined default position. And because Nietzsche was so passionate an atheist, I had my defences down to his unusually intense religiosity and elliptical desire for salvation. Which, I suppose, is how the question of God crept under my intellectual radar.

Nietzsche hated Christianity with all the intensity of someone who had once been caught up in its workings, but he would have equally loathed the high priests of new atheism and their overwhelming sense of intellectual superiority. "How much boundlessly stupid naivety is there in the scholar's belief in his superiority, in the simple, unsuspecting certainty with which his instincts treat the religious man as inferior and a lower type which he himself has evolved above and beyond", he wrote. Nietzsche's big idea goes much deeper than a belief that there is no God. His extraordinary project was to design a form of redemption for a world beyond belief. And to this extent he remained profoundly pious until his dying day.

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