Existentialism is one the most celebrated and promoted school of thought in the area of philosophy, humanities, and in the liberal arts. It started in the late 19th up to the 20th century, and was considered as a revolt or reaction against Hegelian philosophy, positivism, and rationalism, for these thoughts only focus on the object of the study neglecting the bigger concern – the subject or the individual.
The movement concerned itself with the terms like individual existence, freedom, choice, angst, meaning, responsibility, authenticity, death, and etc. Bottom line, it funneled down to the essence and the role of man: what does it mean to be a human being?
Although it navigated through those terms, it was nevertheless divided between two opposing classes – the religious oriented existentialism and the atheistic or naturalistic existentialism.
The Danish philosopher Søren Kierkegaard belonged to the former. Although he died early at the age of forty-two as a “lonely and frustrated man in obscurity in the middle of the nineteenth century,” he was nonetheless heralded as the founder of the existentialist movement, because of the impact of his writings posthumously. And among his several works, three became monumental in the existentialist movement: Either/Or (1843), Philosophical Fragments (1844), and The Sickness unto Death (1849).
But for the sake of this short article, allow me to lay out only three things we can learn from Kierkegaard:
The “learner” experiences “a sickness of the spirit unto death.”
In atheistic existentialism, they widely recognize the meaninglessness of life. There’s no ultimate meaning in life. You’ve got to make your own! However, that resolution also leads to emptiness. To take an instance, consider the Nobel Prize Winner in Literature Albert Camus who began one of his essay with this depressing thought:
There is only one really serious philosophical question, and that is suicide. Judging whether life is or not worth living amounts to answering the fundamental question of philosophy. 
Sad, isn’t it?
On the other hand, Kierkegaard wrote differently. In his writings, he called man as the learner. And that there’s a serious problem in the learner that causes him to experience the angst. Contrary to what the atheist existentialists says, the learner “is born in sin and as a sinner.”  This is where his meaninglessness comes from. And this is what makes him anxious about life and his impending death. “For the learner is in the state of Error.”  Here it is evident that Kierkegaard looked at the human phenomena in the lens of Christianity.
Kierkegaard then laid out three stages of the learner’s life: aesthetic, ethical, and religious – where the first two are dreary. 
The first stage is where the learner solely pursues his sensuality. He is but driven by his passions alone. And his most devastating enemy, unfortunately, is boredom. The second stage, on the other hand, is where learner begins to concentrate on his self-development, and his pursuit of happiness with moral virtue. But still, these two ends up with gnawing despair and emptiness. Why? Because there’s more to man than his physical and moral components. He experiences guilt. For he is in the state of Error or sin.
The learner’s need of the “Teacher”
The learner, unfortunately being in the state of error, is “not able to set himself free.” For he doesn’t even know that he is in the state of Error in the first place, although he experiences a serious kind of guilt. In other words, liberation “is something that no human being can do.”
Given that problem, this is where the role of the Teacher can shed light. “Now if the learner is to acquire the truth, the Teacher must bring it to him.” The Teacher gives “him the condition necessary for understanding it.”
This Teacher of course is God.
The Teacher is…God himself, who in acting as an occasion prompts the learner to recall that he is in Error, and that by reason of his own guilt…. [Liberation] must be done by God himself.
For this very reason, the Teacher is also the Saviour and Redeemer:
What now shall we call such a Teacher, one who restores the lost condition and gives the learner the Truth? Let us call him Saviour, for he saves the learner from his bondage and from himself; let us call him Redeemer, for he redeems the learner from captivity into which he had plunged himself, and no captivity is so terrible and so impossible to break, as that in which the individual keeps himself.
The Teacher of course is not from the world. But who revealed himself to the world in the Moment. He revealed himself in the Fullness of Time, which is very “decisive and filled with the Eternal.”
But why does the Teacher need to reveal himself?
It’s because of love!
Moved by love…God…thus eternally resolved to reveal himself. But as love is the motive so love must also be the end.
And the same Teacher took the wildest position, that is, in be become a servant:
In order that the union may be brought about, the God must therefore become the equal of such a one, and so he will appear in the likeness of the humblest. But the humblest is one who must serve others, and the God will therefore appear in the form of a servant. But this servant-form is no mere outer garment, like the king’s beggar-cloak, which therefore flutters loosely about him and betrays the king; it is not like the filmy summer-cloak of Socrates, which though woven of nothing yet both conceals and reveals. It is his true form and figure. For this is the unfathomable nature of love, that it desires equality with the beloved, not in jest merely, but in earnest and truth. And it is the omnipotence of the love which is so resolved that it is able to accomplish its purpose, which neither Socrates nor the king could do, whence their assumed figures constituted after all a kind of deceit. 
The learner must know that “subjectivity is truth”
This is where Kierkegaard is often misunderstood as advocating relativism. However, that is far from his writings. Notice that the movement concerns itself with the existing individual, i.e., the subject. And of the role of the learner in this world, and in relation to the truth.
Kierkegaard was not opposed to reason, but to rationalism and Hegelian philosophy, who ends up reducing everything to reason while neglecting the existing individual in terms of life’s meaning and ethics. Because according to Kierkegaard, knowledge is not something to be reduced by mere discursive and dialectical analysis, but by living it in life.
So when he said that subjectivity is truth, he is not saying that truth is subjective – but it must be subjectively pursued. That is, the learner is subjectively or personally accountable for his authentic relationship (or the lack thereof) towards the Truth, that is, towards God the Savior, Redeemer, and the Judge.
Kierkegaard’s writings appeal to the reader’s imagination in presenting the Christian truths. This was also true for C. S. Lewis, if I’m not mistaken, who may have been also influenced by Kierkegaard.
So if you want a philosopher that will stir up your imagination towards the truth, I encourage you to include Kierkegaard’s works in your reading list.
 Kenneth Samples, “Christian Thinkers 101: A Crash Course on Søren Kierkegaard.” Available online at https://reflectionsbyken.wordpress.com/2016/07/05/christian-thinkers-101-a-crash-course-on-soren-kierkegaard/
 Albert Camus, The Myth of Sisyphus and Other Essays (New York: Vintage, 1991), 3.
 Soren Kierkegaard, Concluding Unscientific Postscript to the Philosophical Crumbs, 175.
 Kierkegaard, Philosophical Fragments (1844).
 Kierkegaard, Either/Or (1843).
 Kierkegaard, Philosophical Fragments.