As Sören Kierkegaard is often called the father of existentialism, I’ll begin discussing his thoughts by considering what that title means.
The term existential has gotten applied rather haphazardly. In my younger days, foreigners in Mexico for the cheap living and easy access to drugs were called existentialistas. The existentialist label has been applied to most every sort of person who opts against traditional values. I suppose those labels refer to the existentialist insistence on individualism based upon individual choice.
To fit my definition of an existentialist, a person needs to earnestly confront the question: “What does it mean to be existing as a human being?” and the corollary questions: “What is right and wrong?” and “What constitutes a meaningful way of life?” Once he or she has sufficiently confronted these questions, the existentialist will feel called to make a decision; to choose between the plausible answers, and then begin to stand with integrity for the chosen answer.
An atheist, Christian, hippie, Buddhist, or believer in any set of values whether established or self-created, can legitimately be considered an existentialist, as long as her or his adherence to the belief system is grounded in choice rather than unconsidered acceptance.
Suppose I grew up in a Christian Science family, base my beliefs on the teaching of Mary Baker Eddy, and have never seriously questioned those teachings. Since I have declined to exercise my right to consider the options and choose between them, I am certainly no existentialist.
Or suppose, after a modest search for answers, I give up the search and consider myself agnostic. Then I am no existentialist, as I have abdicated my right to vigorously pursue answers then make a conscious and informed choice.
So an existentialist is someone who earnestly seeks to learn the meaning of life then chooses between alternative answers and determines to live in accord with the chosen answer and its implications.
An existentialist worth admiring would also accept responsibility for his or her actions and might live by the mottos: full speed ahead; no turning back; no compromise; no excuses. For an example, read No Compromise, the life story of Christian musician Keith Green.
Kierkegaard framed the decision we now call existentialist as the choice for or against Christ. A line is drawn. On the one side is self-concern. On the other side is love.