Instead of aiming to provide arguments for the truth of Christianity, Negative Apologetics examines the disastrous consequences that would result if Christianity were false. In this sense, Negative Apologetics resembles existentialism. The Danish existentialist Soren Kierkegaard (1813-1855) famously thought that life is lived on three different stages or planes: Aesthetic, Ethical, and Religious. Kierkegaard’s apologetic is one of the best exemplifications of Negative Apologetics. We’ll examine each to see why.
Stage 1: Aesthetic
The first stage on life’s way, according to Kierkegaard, is the Aesthetic. This is the stage of sensuality and pleasure. The aesthete seeks to defeat boredom by maximizing pleasure. Man at this stage needn’t be hedonistic; he can be cultivated and reflective. Nevertheless, life revolves around sex, money, art, music, anything that brings personal enjoyment. He is ruled by his passions.
The problem is that ‘this-worldly pleasure’ is ephemeral. A life defined by fleeting pleasures is one in constant motion. There’s no solid framework from which to operate. Decision making is built on a foundation of incoherence and inconsistency. Eventually the pleasures of aesthetics wear thin and man becomes bored of fighting boredom. Despair, rather than happiness, is found at this stage.
Stage 2: Ethical
The second stage on life’s way is the Ethical. When the pleasures of aesthetics wear thin, one must seek ethical pleasures instead. The ethical life is built on a foundation of morality. Life is no longer about satisfying personal passions, but rather about satisfying the demands of moral obligations (be it absolute or cultural). At this stage, moral conformity becomes the greatest good.
The problem here is that a transitional leap from the aesthetic to the ethical ends in a worse kind of dissatisfaction than boredom. In the second stage of life, man learns that he cannot live up to the demands of morality. The more he tries to conform and do what is required of him, the more he is made painfully aware that he cannot. The ethical life ends in crushing guilt and utter despair.
Stage 3: Religious
The third and most progressive stage, according to Kierkegaard, is the Religious. Once we understand that we are destined to sin, we can understand there’s only one way out. Through faith, we can come to embrace forgiveness of sin and enter into a personal relationship with God. “If we can accept God’s forgiveness, sincerely, inwardly, contritely, with gratitude and hope, then we open ourselves to the joyous prospect of beginning anew.” (source)
Some believe that for Kierkegaard, the transition from the ethical stage of life to the religious is a leap of faith, a criterion-less choice. This is because Christian dogma is characterized by absurdity . Unless man makes this criterion-less leap, his own existence will be characterized by absurdity. His life will remain in utter despair.
Kierkegaard was in essence saying, “In this life there are two choices: Either live a life of despair (stages 1&2), or embrace the absurd (stage 3) and live a life of fulfillment.”
The first important thing to note in our assessment is that, as explained above, Kierkegaard believed that “reason” and faith were in some way at odds with one another . While there were prominent Christians that held a similar view in the past (see Wittgenstein, Pascal, and William James), it isn’t endorsed by many contemporary Christian philosophers. So even if Kierkegaard actually thought that the religious stage of life requires embracing the logically absurd, that isn’t something Christians are necessarily committed to (this is also relevant).
The second important observation is that the first 2 stages are compatible with atheistic moral realism. In other words, Kierkegaard’s Negative Apologetic doesn’t depend on establishing that atheism entails moral relativism; it’s just as strong against the atheist that affirms objective moral values and duties. So even if the intrinsic value of human beings can be squared with blind evolutionary forces, the atheistic life (characterized by stages 1&2) still ends in despair. Aesthetic pleasures in life eventually grow thin. A life built on fleeting pleasures is bound for incoherence and boredom. Likewise, the person that consistently and pursuantly conforms to the standards of morality, even on atheism, will inevitably discover that it can’t actually be done. The ethical life is impossible to live.
Whatever stage the atheist finds themselves in, the only solution is the embrace of Christianity. However, and importantly, this needn’t be at the cost of embracing “absurdity.” There are good reasons to think that Christianity is true (see here and here).
Kierkegaard’s Negative Apologetic closely resembles a common way of presenting the Gospel. In Tactics, Greg Koukl presents the Gospel to a lawyer by (a) establishing the belief that wrong-doing deserves punishment and (b) getting the lawyer to admit that he’d done wrong things and therefore deserved punishment. Christianity provides a succinct solution to this problem. Jesus came and died for us, He bore our sins and endured the pain and suffering we deserved as punishment so that we can be reconciled with the Father. Kierkegaard called this absurdity, I call it love.
 What precisely Kierkegaard meant by the term “absurd” is a matter of debate. Is faith reasonless? Is faith above reason? It is beyond reason? For more, check this book out.