Kierkegaard is the guy you have to read first if you want to understand existentialism, because everyone who came after him — Nietzsche, Sartre, Camus, all of them — was either building on him or arguing with him. He died in 1855, before the word "existentialism" was even coined, but he's the patient zero of the whole thing.
He was born in Copenhagen in 1813, into a wealthy and intensely religious family. His father, Michael Kierkegaard, had risen from poverty to become a successful merchant. He'd also, as a boy, once cursed God in a moment of bitter despair on a barren Jutland heath. He'd never gotten over the guilt of it. He believed his entire family was under a curse for that single moment of blasphemy. He raised Søren and his siblings in an atmosphere of constant religious dread.
Søren was the youngest of seven kids. By the time he was 21, five of his six siblings and his mother were dead. Only his older brother Peter survived. The dad's prediction — that the curse would take everyone — seemed to be coming true. Søren grew up convinced he wouldn't live past 33, the age Christ died. (He almost made it. He died at 42.)
When he was 24, he met a 14-year-old girl named Regine Olsen at a social gathering. He fell for her immediately. They got engaged when she was 17. He spent the next year of the engagement increasingly tortured by the conviction that he was too dark, too melancholic, too internally damaged to give her the cheerful normal married life she deserved. So he ended it. Brutally. He sent her back the engagement ring with a cold letter and then deliberately acted like a heartless playboy in public so she'd hate him and move on.
It almost worked. She did, eventually, marry someone else. But Kierkegaard never got over her. Nearly every book he wrote afterward had her ghost in it. He'd dedicate things to "Her." He'd encode messages he hoped she'd read. When he died 14 years later he left her his entire estate (her husband refused it). The breakup was the wound he wrote out of for the rest of his life.
His writing exploded out of that wound. Between 1843 and 1855 he produced something like twenty major books and thousands of journal pages. A lot of it was published under fake names — Johannes Climacus, Anti-Climacus, Constantin Constantius, Hilarius Bookbinder, the names get weirder. Each pseudonym was its own carefully constructed character with its own philosophy. He was running a one-man dialectical war between different views of life inside his own books.
His central idea is this: existence is something you have to choose, not something you're handed. Other philosophers had been arguing for centuries about what humans are — the rational animal, the social animal, whatever. Kierkegaard goes: that whole question is the wrong question. You're not a thing that has a fixed essence. You're a project. You become someone by committing to things. The choice precedes the self.
Anxiety is the dizziness of freedom.
That line might be the most famous thing he wrote, and it's still the best one-sentence diagnosis of modern life. We're free. Genuinely free, in a way no previous generation has been free. We can choose our religion, our partner, our career, our gender, our city, our entire identity. And that freedom, instead of feeling great, makes us anxious. Because every choice forecloses other choices. Every commitment kills off versions of yourself that won't get to exist. The dizziness of having to pick is its own kind of suffering, and Kierkegaard saw it 180 years ago.
His map of human life had three stages. The aesthetic stage is where most modern people live — chasing pleasures, distractions, sensations, novelty. Always moving on before the boredom catches up. The ethical stage is when you commit to something — a marriage, a career, a moral code — and accept the constraints that come with it. The religious stage is harder to describe, but it's something like a leap into faith that goes beyond reason — committing to something you can't prove and that might even look absurd from the outside, but that gives life a different gravity.
The famous example he kept coming back to was Abraham, who in the Old Testament is asked by God to sacrifice his son Isaac. From an ethical standpoint, what Abraham agrees to do is monstrous. There's no rational defense of it. But Kierkegaard is fascinated by it because Abraham's faith is operating on a level that ethics can't reach. He calls this the teleological suspension of the ethical — the moment when faith asks something of you that ethics would forbid. The leap of faith is into territory that doesn't make sense, and that's the whole point.
Life can only be understood backwards; but it must be lived forwards.
His last years got hard. He spent his entire inheritance funding his own books. He got into a vicious public feud with the satirical newspaper The Corsair, which mocked him relentlessly. Then he started attacking the Danish State Church publicly — arguing that it had turned Christianity into a comfortable middle-class lifestyle instead of the radical, terrifying commitment it was supposed to be. He published a series of pamphlets called The Moment in his final months, openly accusing the bishops of betraying Christ for respectability.
He collapsed in the street in October 1855, was hospitalized, and died about a month later. He was 42. His brother visited him in the hospital and Søren refused to take communion from him because his brother was a state-church priest. He died, as he'd lived, picking a fight.
The reason Kierkegaard still matters to me is that he was the first person to take the actual texture of modern psychological life seriously as a philosophical subject. The anxiety, the despair, the sense that the choice itself is overwhelming, the suspicion that all the standard answers are slightly fake — those are his territory. Read him and you find someone 180 years ago describing exactly what it feels like to be 28 and lost and wired and Catholic-guilty and stuck in your own head. Turns out modern existential dread is older than we thought.